Functional programming is an approach to software development based around the evaluation of functions. Like mathematics, functions in programming map input to output to produce a result. You can combine basic functions in many ways to build more and more complex programs.
Functional programming follows a few core principles:
Functions are independent from the state of the program or global variables. They only depend on the arguments passed into them to make a calculation
Functions try to limit any changes to the state of the program and avoid changes to the global objects holding data
Functions have minimal side effects in the program
The functional programming software development approach breaks a program into small, testable parts. This section covers basic functional programming principles in JavaScript.
Functional programming is a style of programming where solutions are simple, isolated functions, without any side effects outside of the function scope.
INPUT -> PROCESS -> OUTPUT
Functional programming is about:
-
Isolated functions - there is no dependence on the state of the program, which includes global variables that are subject to change
-
Pure functions - the same input always gives the same output
-
Functions with limited side effects - any changes, or mutations, to the state of the program outside the function are carefully controlled
The members of freeCodeCamp happen to love tea.
In the code editor, the prepareTea and getTea functions are already defined for you. Call the getTea function to get 40 cups of tea for the team, and store them in the tea4TeamFCC variable.
/**
* A long process to prepare tea.
* @return {string} A cup of tea.
**/
const prepareTea = () => 'greenTea';
/**
* Get given number of cups of tea.
* @param {number} numOfCups Number of required cups of tea.
* @return {Array<string>} Given amount of tea cups.
**/
const getTea = (numOfCups) => {
const teaCups = [];
for(let cups = 1; cups <= numOfCups; cups += 1) {
const teaCup = prepareTea();
teaCups.push(teaCup);
}
return teaCups;
};
// Add your code below this line
const tea4TeamFCC = getTea(40); // :(
// Add your code above this line
console.log(tea4TeamFCC);
The FCC Team had a mood swing and now wants two types of tea: green tea and black tea. General Fact: Client mood swings are pretty common.
With that information, we'll need to revisit the getTea function from last challenge to handle various tea requests. We can modify getTea to accept a function as a parameter to be able to change the type of tea it prepares. This makes getTea more flexible, and gives the programmer more control when client requests change.
But first, let's cover some functional terminology:
Callbacks are the functions that are slipped or passed into another function to decide the invocation of that function. You may have seen them passed to other methods, for example in filter, the callback function tells JavaScript the criteria for how to filter an array.
Functions that can be assigned to a variable, passed into another function, or returned from another function just like any other normal value, are called first class functions. In JavaScript, all functions are first class functions.
The functions that take a function as an argument, or return a function as a return value are called higher order functions.
When the functions are passed in to another function or returned from another function, then those functions which gets passed in or returned can be called a lambda.
Prepare 27 cups of green tea and 13 cups of black tea and store them in tea4GreenTeamFCC and tea4BlackTeamFCC variables, respectively. Note that the getTea function has been modified so it now takes a function as the first argument.
Note: The data (the number of cups of tea) is supplied as the last argument. We'll discuss this more in later lessons.
/**
* A long process to prepare green tea.
* @return {string} A cup of green tea.
**/
const prepareGreenTea = () => 'greenTea';
/**
* A long process to prepare black tea.
* @return {string} A cup of black tea.
**/
const prepareBlackTea = () => 'blackTea';
/**
* Get given number of cups of tea.
* @param {function():string} prepareTea The type of tea preparing function.
* @param {number} numOfCups Number of required cups of tea.
* @return {Array<string>} Given amount of tea cups.
**/
const getTea = (prepareTea, numOfCups) => {
const teaCups = [];
for(let cups = 1; cups <= numOfCups; cups += 1) {
const teaCup = prepareTea();
teaCups.push(teaCup);
}
return teaCups;
};
// Add your code below this line
const tea4GreenTeamFCC = getTea(prepareGreenTea, 27); // :(
const tea4BlackTeamFCC = getTea(prepareBlackTea, 13); // :(
// Add your code above this line
console.log(
tea4GreenTeamFCC,
tea4BlackTeamFCC
);
Functional programming is a good habit. It keeps your code easy to manage, and saves you from sneaky bugs. But before we get there, let's look at an imperative approach to programming to highlight where you may have issues.
In English (and many other languages), the imperative tense is used to give commands. Similarly, an imperative style in programming is one that gives the computer a set of statements to perform a task.
Often the statements change the state of the program, like updating global variables. A classic example is writing a for loop that gives exact directions to iterate over the indices of an array.
In contrast, functional programming is a form of declarative programming. You tell the computer what you want done by calling a method or function.
JavaScript offers many predefined methods that handle common tasks so you don't need to write out how the computer should perform them. For example, instead of using the for loop mentioned above, you could call the map method which handles the details of iterating over an array. This helps to avoid semantic errors, like the "Off By One Errors" that were covered in the Debugging section.
Consider the scenario: you are browsing the web in your browser, and want to track the tabs you have opened. Let's try to model this using some simple object-oriented code.
A Window object is made up of tabs, and you usually have more than one Window open. The titles of each open site in each Window object is held in an array. After working in the browser (opening new tabs, merging windows, and closing tabs), you want to print the tabs that are still open. Closed tabs are removed from the array and new tabs (for simplicity) get added to the end of it.
The code editor shows an implementation of this functionality with functions for tabOpen(), tabClose(), and join(). The array tabs is part of the Window object that stores the name of the open pages.
Instructions Run the code in the editor. It's using a method that has side effects in the program, causing incorrect output. The final list of open tabs should be ['FB', 'Gitter', 'Reddit', 'Twitter', 'Medium', 'new tab', 'Netflix', 'YouTube', 'Vine', 'GMail', 'Work mail', 'Docs', 'freeCodeCamp', 'new tab'] but the output will be slightly different.
Work through the code and see if you can figure out the problem, then advance to the next challenge to learn more.
// tabs is an array of titles of each site open within the window
var Window = function(tabs) {
this.tabs = tabs; // we keep a record of the array inside the object
};
// When you join two windows into one window
Window.prototype.join = function (otherWindow) {
this.tabs = this.tabs.concat(otherWindow.tabs);
return this;
};
// When you open a new tab at the end
Window.prototype.tabOpen = function (tab) {
this.tabs.push('new tab'); // let's open a new tab for now
return this;
};
// When you close a tab
Window.prototype.tabClose = function (index) {
var tabsBeforeIndex = this.tabs.splice(0, index); // get the tabs before the tab
var tabsAfterIndex = this.tabs.splice(index); // get the tabs after the tab
this.tabs = tabsBeforeIndex.concat(tabsAfterIndex); // join them together
return this;
};
// Let's create three browser windows
var workWindow = new Window(['GMail', 'Inbox', 'Work mail', 'Docs', 'freeCodeCamp']); // Your mailbox, drive, and other work sites
var socialWindow = new Window(['FB', 'Gitter', 'Reddit', 'Twitter', 'Medium']); // Social sites
var videoWindow = new Window(['Netflix', 'YouTube', 'Vimeo', 'Vine']); // Entertainment sites
// Now perform the tab opening, closing, and other operations
var finalTabs = socialWindow
.tabOpen() // Open a new tab for cat memes
.join(videoWindow.tabClose(2)) // Close third tab in video window, and join
.join(workWindow.tabClose(1).tabOpen());
alert(finalTabs.tabs);
If you haven't already figured it out, the issue in the previous challenge was with the splice call in the tabClose() function. Unfortunately, splice changes the original array it is called on, so the second call to it used a modified array, and gave unexpected results.
This is a small example of a much larger pattern - you call a function on a variable, array, or an object, and the function changes the variable or something in the object.
One of the core principle of functional programming is to not change things. Changes lead to bugs. It's easier to prevent bugs knowing that your functions don't change anything, including the function arguments or any global variable.
The previous example didn't have any complicated operations but the splice method changed the original array, and resulted in a bug.
Recall that in functional programming, changing or altering things is called mutation, and the outcome is called a side effect. A function, ideally, should be a pure function, meaning that it does not cause any side effects.
Let's try to master this discipline and not alter any variable or object in our code.
Fill in the code for the function incrementer so it returns the value of the global variable fixedValue increased by one.
// the global variable
var fixedValue = 4;
function incrementer () {
// Add your code below this line
return fixedValue + 1;
// Add your code above this line
}
var newValue = incrementer(); // Should equal 5
console.log(fixedValue); // Should print 4
The last challenge was a step closer to functional programming principles, but there is still something missing.
We didn't alter the global variable value, but the function incrementer would not work without the global variable fixedValue being there.
Another principle of functional programming is to always declare your dependencies explicitly. This means if a function depends on a variable or object being present, then pass that variable or object directly into the function as an argument.
There are several good consequences from this principle. The function is easier to test, you know exactly what input it takes, and it won't depend on anything else in your program.
This can give you more confidence when you alter, remove, or add new code. You would know what you can or cannot change and you can see where the potential traps are.
Finally, the function would always produce the same output for the same set of inputs, no matter what part of the code executes it.
Let's update the incrementer function to clearly declare its dependencies.
Write the incrementer function so it takes an argument, and then increases the value by one.
// the global variable
var fixedValue = 4;
// Add your code below this line
function incrementer (number) {
return number + 1;
// Add your code above this line
}
var newValue = incrementer(fixedValue); // Should equal 5
console.log(fixedValue); // Should print 4
So far, we have seen two distinct principles for functional programming:
-
Don't alter a variable or object - create new variables and objects and return them if need be from a function.
-
Declare function arguments - any computation inside a function depends only on the arguments, and not on any global object or variable.
Adding one to a number is not very exciting, but we can apply these principles when working with arrays or more complex objects.
Refactor (rewrite) the code so the global array bookList is not changed inside either function. The add function should add the given bookName to the end of an array. The remove function should remove the given bookName from an array. Both functions should return an array, and any new parameters should be added before the bookName one.
// the global variable
var bookList = ["The Hound of the Baskervilles", "On The Electrodynamics of Moving Bodies", "Philosophiæ Naturalis Principia Mathematica", "Disquisitiones Arithmeticae"];
/* This function should add a book to the list and return the list */
// New parameters should come before the bookName one
// Add your code below this line
function add (bookArr, bookName) {
return [...bookArr, bookName];
// Add your code above this line
}
/* This function should remove a book from the list and return the list */
// New parameters should come before the bookName one
// Add your code below this line
function remove (bookArr, bookName) {
if ([...bookArr].indexOf(bookName) >= 0) {
return [...bookArr].filter(bookname => bookname !== bookName);
// Add your code above this line
}
}
var newBookList = add(bookList, 'A Brief History of Time');
var newerBookList = remove(bookList, 'On The Electrodynamics of Moving Bodies');
var newestBookList = remove(add(bookList, 'A Brief History of Time'), 'On The Electrodynamics of Moving Bodies');
console.log(bookList);
So far we have learned to use pure functions to avoid side effects in a program. Also, we have seen the value in having a function only depend on its input arguments.
This is only the beginning. As its name suggests, functional programming is centered around a theory of functions.
It would make sense to be able to pass them as arguments to other functions, and return a function from another function. Functions are considered First Class Objects in JavaScript, which means they can be used like any other object. They can be saved in variables, stored in an object, or passed as function arguments.
Let's start with some simple array functions, which are methods on the array object prototype. In this exercise we are looking at Array.prototype.map(), or more simply map.
Remember that the map method is a way to iterate over each item in an array. It creates a new array (without changing the original one) after applying a callback function to every element.
The watchList array holds objects with information on several movies. Use map to pull the title and rating from watchList and save the new array in the rating variable. The code in the editor currently uses a for loop to do this, replace the loop functionality with your map expression.
// the global variable
var watchList = [
{
"Title": "Inception",
"Year": "2010",
"Rated": "PG-13",
"Released": "16 Jul 2010",
"Runtime": "148 min",
"Genre": "Action, Adventure, Crime",
"Director": "Christopher Nolan",
"Writer": "Christopher Nolan",
"Actors": "Leonardo DiCaprio, Joseph Gordon-Levitt, Ellen Page, Tom Hardy",
"Plot": "A thief, who steals corporate secrets through use of dream-sharing technology, is given the inverse task of planting an idea into the mind of a CEO.",
"Language": "English, Japanese, French",
"Country": "USA, UK",
"Awards": "Won 4 Oscars. Another 143 wins & 198 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BMjAxMzY3NjcxNF5BMl5BanBnXkFtZTcwNTI5OTM0Mw@@._V1_SX300.jpg",
"Metascore": "74",
"imdbRating": "8.8",
"imdbVotes": "1,446,708",
"imdbID": "tt1375666",
"Type": "movie",
"Response": "True"
},
{
"Title": "Interstellar",
"Year": "2014",
"Rated": "PG-13",
"Released": "07 Nov 2014",
"Runtime": "169 min",
"Genre": "Adventure, Drama, Sci-Fi",
"Director": "Christopher Nolan",
"Writer": "Jonathan Nolan, Christopher Nolan",
"Actors": "Ellen Burstyn, Matthew McConaughey, Mackenzie Foy, John Lithgow",
"Plot": "A team of explorers travel through a wormhole in space in an attempt to ensure humanity's survival.",
"Language": "English",
"Country": "USA, UK",
"Awards": "Won 1 Oscar. Another 39 wins & 132 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BMjIxNTU4MzY4MF5BMl5BanBnXkFtZTgwMzM4ODI3MjE@._V1_SX300.jpg",
"Metascore": "74",
"imdbRating": "8.6",
"imdbVotes": "910,366",
"imdbID": "tt0816692",
"Type": "movie",
"Response": "True"
},
{
"Title": "The Dark Knight",
"Year": "2008",
"Rated": "PG-13",
"Released": "18 Jul 2008",
"Runtime": "152 min",
"Genre": "Action, Adventure, Crime",
"Director": "Christopher Nolan",
"Writer": "Jonathan Nolan (screenplay), Christopher Nolan (screenplay), Christopher Nolan (story), David S. Goyer (story), Bob Kane (characters)",
"Actors": "Christian Bale, Heath Ledger, Aaron Eckhart, Michael Caine",
"Plot": "When the menace known as the Joker wreaks havoc and chaos on the people of Gotham, the caped crusader must come to terms with one of the greatest psychological tests of his ability to fight injustice.",
"Language": "English, Mandarin",
"Country": "USA, UK",
"Awards": "Won 2 Oscars. Another 146 wins & 142 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BMTMxNTMwODM0NF5BMl5BanBnXkFtZTcwODAyMTk2Mw@@._V1_SX300.jpg",
"Metascore": "82",
"imdbRating": "9.0",
"imdbVotes": "1,652,832",
"imdbID": "tt0468569",
"Type": "movie",
"Response": "True"
},
{
"Title": "Batman Begins",
"Year": "2005",
"Rated": "PG-13",
"Released": "15 Jun 2005",
"Runtime": "140 min",
"Genre": "Action, Adventure",
"Director": "Christopher Nolan",
"Writer": "Bob Kane (characters), David S. Goyer (story), Christopher Nolan (screenplay), David S. Goyer (screenplay)",
"Actors": "Christian Bale, Michael Caine, Liam Neeson, Katie Holmes",
"Plot": "After training with his mentor, Batman begins his fight to free crime-ridden Gotham City from the corruption that Scarecrow and the League of Shadows have cast upon it.",
"Language": "English, Urdu, Mandarin",
"Country": "USA, UK",
"Awards": "Nominated for 1 Oscar. Another 15 wins & 66 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BNTM3OTc0MzM2OV5BMl5BanBnXkFtZTYwNzUwMTI3._V1_SX300.jpg",
"Metascore": "70",
"imdbRating": "8.3",
"imdbVotes": "972,584",
"imdbID": "tt0372784",
"Type": "movie",
"Response": "True"
},
{
"Title": "Avatar",
"Year": "2009",
"Rated": "PG-13",
"Released": "18 Dec 2009",
"Runtime": "162 min",
"Genre": "Action, Adventure, Fantasy",
"Director": "James Cameron",
"Writer": "James Cameron",
"Actors": "Sam Worthington, Zoe Saldana, Sigourney Weaver, Stephen Lang",
"Plot": "A paraplegic marine dispatched to the moon Pandora on a unique mission becomes torn between following his orders and protecting the world he feels is his home.",
"Language": "English, Spanish",
"Country": "USA, UK",
"Awards": "Won 3 Oscars. Another 80 wins & 121 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BMTYwOTEwNjAzMl5BMl5BanBnXkFtZTcwODc5MTUwMw@@._V1_SX300.jpg",
"Metascore": "83",
"imdbRating": "7.9",
"imdbVotes": "876,575",
"imdbID": "tt0499549",
"Type": "movie",
"Response": "True"
}
];
// Add your code below this line
const rating = watchList.map(el => ({title: el.Title, rating: el.imdbRating}));
// Add your code above this line
console.log(rating);
As you have seen from applying Array.prototype.map(), or simply map() earlier, the map method returns an array of the same length as the one it was called on. It also doesn't alter the original array, as long as its callback function doesn't.
In other words, map is a pure function, and its output depends solely on its inputs. Plus, it takes another function as its argument.
It would teach us a lot about map to try to implement a version of it that behaves exactly like the Array.prototype.map() with a for loop or Array.prototype.forEach().
Note: A pure function is allowed to alter local variables defined within its scope, although, it's preferable to avoid that as well.
Write your own Array.prototype.myMap(), which should behave exactly like Array.prototype.map(). You may use a for loop or the forEach method.
// the global Array
var s = [23, 65, 98, 5];
Array.prototype.myMap = function(callback){
var newArray = [];
// Add your code below this line
this.forEach(el => newArray.push(callback(el)))
// Add your code above this line
return newArray;
};
var new_s = s.myMap(function(item){
return item * 2;
});
Another useful array function is Array.prototype.filter(), or simply filter(). The filter method returns a new array which is at most as long as the original array, but usually has fewer items.
Filter doesn't alter the original array, just like map. It takes a callback function that applies the logic inside the callback on each element of the array. If an element returns true based on the criteria in the callback function, then it is included in the new array.
The variable watchList holds an array of objects with information on several movies. Use a combination of filter and map to return a new array of objects with only title and rating keys, but where imdbRating is greater than or equal to 8.0. Note that the rating values are saved as strings in the object and you may want to convert them into numbers to perform mathematical operations on them.
// the global variable
var watchList = [
{
"Title": "Inception",
"Year": "2010",
"Rated": "PG-13",
"Released": "16 Jul 2010",
"Runtime": "148 min",
"Genre": "Action, Adventure, Crime",
"Director": "Christopher Nolan",
"Writer": "Christopher Nolan",
"Actors": "Leonardo DiCaprio, Joseph Gordon-Levitt, Ellen Page, Tom Hardy",
"Plot": "A thief, who steals corporate secrets through use of dream-sharing technology, is given the inverse task of planting an idea into the mind of a CEO.",
"Language": "English, Japanese, French",
"Country": "USA, UK",
"Awards": "Won 4 Oscars. Another 143 wins & 198 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BMjAxMzY3NjcxNF5BMl5BanBnXkFtZTcwNTI5OTM0Mw@@._V1_SX300.jpg",
"Metascore": "74",
"imdbRating": "8.8",
"imdbVotes": "1,446,708",
"imdbID": "tt1375666",
"Type": "movie",
"Response": "True"
},
{
"Title": "Interstellar",
"Year": "2014",
"Rated": "PG-13",
"Released": "07 Nov 2014",
"Runtime": "169 min",
"Genre": "Adventure, Drama, Sci-Fi",
"Director": "Christopher Nolan",
"Writer": "Jonathan Nolan, Christopher Nolan",
"Actors": "Ellen Burstyn, Matthew McConaughey, Mackenzie Foy, John Lithgow",
"Plot": "A team of explorers travel through a wormhole in space in an attempt to ensure humanity's survival.",
"Language": "English",
"Country": "USA, UK",
"Awards": "Won 1 Oscar. Another 39 wins & 132 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BMjIxNTU4MzY4MF5BMl5BanBnXkFtZTgwMzM4ODI3MjE@._V1_SX300.jpg",
"Metascore": "74",
"imdbRating": "8.6",
"imdbVotes": "910,366",
"imdbID": "tt0816692",
"Type": "movie",
"Response": "True"
},
{
"Title": "The Dark Knight",
"Year": "2008",
"Rated": "PG-13",
"Released": "18 Jul 2008",
"Runtime": "152 min",
"Genre": "Action, Adventure, Crime",
"Director": "Christopher Nolan",
"Writer": "Jonathan Nolan (screenplay), Christopher Nolan (screenplay), Christopher Nolan (story), David S. Goyer (story), Bob Kane (characters)",
"Actors": "Christian Bale, Heath Ledger, Aaron Eckhart, Michael Caine",
"Plot": "When the menace known as the Joker wreaks havoc and chaos on the people of Gotham, the caped crusader must come to terms with one of the greatest psychological tests of his ability to fight injustice.",
"Language": "English, Mandarin",
"Country": "USA, UK",
"Awards": "Won 2 Oscars. Another 146 wins & 142 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BMTMxNTMwODM0NF5BMl5BanBnXkFtZTcwODAyMTk2Mw@@._V1_SX300.jpg",
"Metascore": "82",
"imdbRating": "9.0",
"imdbVotes": "1,652,832",
"imdbID": "tt0468569",
"Type": "movie",
"Response": "True"
},
{
"Title": "Batman Begins",
"Year": "2005",
"Rated": "PG-13",
"Released": "15 Jun 2005",
"Runtime": "140 min",
"Genre": "Action, Adventure",
"Director": "Christopher Nolan",
"Writer": "Bob Kane (characters), David S. Goyer (story), Christopher Nolan (screenplay), David S. Goyer (screenplay)",
"Actors": "Christian Bale, Michael Caine, Liam Neeson, Katie Holmes",
"Plot": "After training with his mentor, Batman begins his fight to free crime-ridden Gotham City from the corruption that Scarecrow and the League of Shadows have cast upon it.",
"Language": "English, Urdu, Mandarin",
"Country": "USA, UK",
"Awards": "Nominated for 1 Oscar. Another 15 wins & 66 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BNTM3OTc0MzM2OV5BMl5BanBnXkFtZTYwNzUwMTI3._V1_SX300.jpg",
"Metascore": "70",
"imdbRating": "8.3",
"imdbVotes": "972,584",
"imdbID": "tt0372784",
"Type": "movie",
"Response": "True"
},
{
"Title": "Avatar",
"Year": "2009",
"Rated": "PG-13",
"Released": "18 Dec 2009",
"Runtime": "162 min",
"Genre": "Action, Adventure, Fantasy",
"Director": "James Cameron",
"Writer": "James Cameron",
"Actors": "Sam Worthington, Zoe Saldana, Sigourney Weaver, Stephen Lang",
"Plot": "A paraplegic marine dispatched to the moon Pandora on a unique mission becomes torn between following his orders and protecting the world he feels is his home.",
"Language": "English, Spanish",
"Country": "USA, UK",
"Awards": "Won 3 Oscars. Another 80 wins & 121 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BMTYwOTEwNjAzMl5BMl5BanBnXkFtZTcwODc5MTUwMw@@._V1_SX300.jpg",
"Metascore": "83",
"imdbRating": "7.9",
"imdbVotes": "876,575",
"imdbID": "tt0499549",
"Type": "movie",
"Response": "True"
}
];
// Add your code below this line
const filteredList = watchList.map(film => ({title: film.Title, rating: film.imdbRating})).filter(film => parseFloat(film.rating) >= 8);
// Add your code above this line
console.log(filteredList);
It would teach us a lot about the filter method if we try to implement a version of it that behaves exactly like Array.prototype.filter(). It can use either a for loop or Array.prototype.forEach().
Note: A pure function is allowed to alter local variables defined within its scope, although, it's preferable to avoid that as well.
Write your own Array.prototype.myFilter(), which should behave exactly like Array.prototype.filter(). You may use a for loop or the Array.prototype.forEach() method.
// the global Array
var s = [23, 65, 98, 5];
Array.prototype.myFilter = function(callback){
var newArray = [];
// Add your code below this line
this.forEach(el => {if(callback(el)) newArray.push(el)});
// Add your code above this line
return newArray;
};
var new_s = s.myFilter(function(item){
return item % 2 === 1;
});
The slice method returns a copy of certain elements of an array. It can take two arguments, the first gives the index of where to begin the slice, the second is the index for where to end the slice (and it's non-inclusive). If the arguments are not provided, the default is to start at the beginning of the array through the end, which is an easy way to make a copy of the entire array. The slice method does not mutate the original array, but returns a new one.
Here's an example:
var arr = ["Cat", "Dog", "Tiger", "Zebra"];
var newArray = arr.slice(1, 3);
// Sets newArray to ["Dog", "Tiger"]
Use the slice method in the sliceArray function to return part of the anim array given the provided beginSlice and endSlice indices. The function should return an array.
function sliceArray(anim, beginSlice, endSlice) {
// Add your code below this line
return anim.slice(beginSlice, endSlice);
// Add your code above this line
}
var inputAnim = ["Cat", "Dog", "Tiger", "Zebra", "Ant"];
sliceArray(inputAnim, 1, 3);
A common pattern while working with arrays is when you want to remove items and keep the rest of the array. JavaScript offers the splice method for this, which takes arguments for the index of where to start removing items, then the number of items to remove. If the second argument is not provided, the default is to remove items through the end. However, the splice method mutates the original array it is called on. Here's an example:
var cities = ["Chicago", "Delhi", "Islamabad", "London", "Berlin"];
cities.splice(3, 1); // Returns "London" and deletes it from the cities array
// cities is now ["Chicago", "Delhi", "Islamabad", "Berlin"]
As we saw in the last challenge, the slice method does not mutate the original array, but returns a new one which can be saved into a variable. Recall that the slice method takes two arguments for the indices to begin and end the slice (the end is non-inclusive), and returns those items in a new array. Using the slice method instead of splice helps to avoid any array-mutating side effects.
Rewrite the function nonMutatingSplice by using slice instead of splice. It should limit the provided cities array to a length of 3, and return a new array with only the first three items.
Do not mutate the original array provided to the function.
function nonMutatingSplice(cities) {
// Add your code below this line
return cities.slice(0,3);
// Add your code above this line
}
var inputCities = ["Chicago", "Delhi", "Islamabad", "London", "Berlin"];
nonMutatingSplice(inputCities);
Concatenation means to join items end to end. JavaScript offers the concat method for both strings and arrays that work in the same way. For arrays, the method is called on one, then another array is provided as the argument to concat, which is added to the end of the first array. It returns a new array and does not mutate either of the original arrays. Here's an example:
[1, 2, 3].concat([4, 5, 6]);
// Returns a new array [1, 2, 3, 4, 5, 6]
Use the concat method in the nonMutatingConcat function to concatenate attach to the end of original. The function should return the concatenated array.
function nonMutatingConcat(original, attach) {
// Add your code below this line
return original.concat(attach);
// Add your code above this line
}
var first = [1, 2, 3];
var second = [4, 5];
nonMutatingConcat(first, second);
Functional programming is all about creating and using non-mutating functions.
The last challenge introduced the concat method as a way to combine arrays into a new one without mutating the original arrays. Compare concat to the push method. Push adds an item to the end of the same array it is called on, which mutates that array. Here's an example:
var arr = [1, 2, 3];
arr.push([4, 5, 6]);
// arr is changed to [1, 2, 3, [4, 5, 6]]
// Not the functional programming way
Concat offers a way to add new items to the end of an array without any mutating side effects.
Change the nonMutatingPush function so it uses concat to add newItem to the end of original instead of push. The function should return an array.
function nonMutatingPush(original, newItem) {
// Add your code below this line
return original.concat(newItem);
// Add your code above this line
}
var first = [1, 2, 3];
var second = [4, 5];
nonMutatingPush(first, second);
Array.prototype.reduce(), or simply reduce(), is the most general of all array operations in JavaScript. You can solve almost any array processing problem using the reduce method.
This is not the case with the filter and map methods since they do not allow interaction between two different elements of the array. For example, if you want to compare elements of the array, or add them together, filter or map could not process that.
The reduce method allows for more general forms of array processing, and it's possible to show that both filter and map can be derived as a special application of reduce.
However, before we get there, let's practice using reduce first.
The variable watchList holds an array of objects with information on several movies. Use reduce to find the average IMDB rating of the movies directed by Christopher Nolan. Recall from prior challenges how to filter data and map over it to pull what you need. You may need to create other variables, but save the final average into the variable averageRating. Note that the rating values are saved as strings in the object and need to be converted into numbers before they are used in any mathematical operations.
// the global variable
var watchList = [
{
"Title": "Inception",
"Year": "2010",
"Rated": "PG-13",
"Released": "16 Jul 2010",
"Runtime": "148 min",
"Genre": "Action, Adventure, Crime",
"Director": "Christopher Nolan",
"Writer": "Christopher Nolan",
"Actors": "Leonardo DiCaprio, Joseph Gordon-Levitt, Ellen Page, Tom Hardy",
"Plot": "A thief, who steals corporate secrets through use of dream-sharing technology, is given the inverse task of planting an idea into the mind of a CEO.",
"Language": "English, Japanese, French",
"Country": "USA, UK",
"Awards": "Won 4 Oscars. Another 143 wins & 198 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BMjAxMzY3NjcxNF5BMl5BanBnXkFtZTcwNTI5OTM0Mw@@._V1_SX300.jpg",
"Metascore": "74",
"imdbRating": "8.8",
"imdbVotes": "1,446,708",
"imdbID": "tt1375666",
"Type": "movie",
"Response": "True"
},
{
"Title": "Interstellar",
"Year": "2014",
"Rated": "PG-13",
"Released": "07 Nov 2014",
"Runtime": "169 min",
"Genre": "Adventure, Drama, Sci-Fi",
"Director": "Christopher Nolan",
"Writer": "Jonathan Nolan, Christopher Nolan",
"Actors": "Ellen Burstyn, Matthew McConaughey, Mackenzie Foy, John Lithgow",
"Plot": "A team of explorers travel through a wormhole in space in an attempt to ensure humanity's survival.",
"Language": "English",
"Country": "USA, UK",
"Awards": "Won 1 Oscar. Another 39 wins & 132 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BMjIxNTU4MzY4MF5BMl5BanBnXkFtZTgwMzM4ODI3MjE@._V1_SX300.jpg",
"Metascore": "74",
"imdbRating": "8.6",
"imdbVotes": "910,366",
"imdbID": "tt0816692",
"Type": "movie",
"Response": "True"
},
{
"Title": "The Dark Knight",
"Year": "2008",
"Rated": "PG-13",
"Released": "18 Jul 2008",
"Runtime": "152 min",
"Genre": "Action, Adventure, Crime",
"Director": "Christopher Nolan",
"Writer": "Jonathan Nolan (screenplay), Christopher Nolan (screenplay), Christopher Nolan (story), David S. Goyer (story), Bob Kane (characters)",
"Actors": "Christian Bale, Heath Ledger, Aaron Eckhart, Michael Caine",
"Plot": "When the menace known as the Joker wreaks havoc and chaos on the people of Gotham, the caped crusader must come to terms with one of the greatest psychological tests of his ability to fight injustice.",
"Language": "English, Mandarin",
"Country": "USA, UK",
"Awards": "Won 2 Oscars. Another 146 wins & 142 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BMTMxNTMwODM0NF5BMl5BanBnXkFtZTcwODAyMTk2Mw@@._V1_SX300.jpg",
"Metascore": "82",
"imdbRating": "9.0",
"imdbVotes": "1,652,832",
"imdbID": "tt0468569",
"Type": "movie",
"Response": "True"
},
{
"Title": "Batman Begins",
"Year": "2005",
"Rated": "PG-13",
"Released": "15 Jun 2005",
"Runtime": "140 min",
"Genre": "Action, Adventure",
"Director": "Christopher Nolan",
"Writer": "Bob Kane (characters), David S. Goyer (story), Christopher Nolan (screenplay), David S. Goyer (screenplay)",
"Actors": "Christian Bale, Michael Caine, Liam Neeson, Katie Holmes",
"Plot": "After training with his mentor, Batman begins his fight to free crime-ridden Gotham City from the corruption that Scarecrow and the League of Shadows have cast upon it.",
"Language": "English, Urdu, Mandarin",
"Country": "USA, UK",
"Awards": "Nominated for 1 Oscar. Another 15 wins & 66 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BNTM3OTc0MzM2OV5BMl5BanBnXkFtZTYwNzUwMTI3._V1_SX300.jpg",
"Metascore": "70",
"imdbRating": "8.3",
"imdbVotes": "972,584",
"imdbID": "tt0372784",
"Type": "movie",
"Response": "True"
},
{
"Title": "Avatar",
"Year": "2009",
"Rated": "PG-13",
"Released": "18 Dec 2009",
"Runtime": "162 min",
"Genre": "Action, Adventure, Fantasy",
"Director": "James Cameron",
"Writer": "James Cameron",
"Actors": "Sam Worthington, Zoe Saldana, Sigourney Weaver, Stephen Lang",
"Plot": "A paraplegic marine dispatched to the moon Pandora on a unique mission becomes torn between following his orders and protecting the world he feels is his home.",
"Language": "English, Spanish",
"Country": "USA, UK",
"Awards": "Won 3 Oscars. Another 80 wins & 121 nominations.",
"Poster": "http://ia.media-imdb.com/images/M/MV5BMTYwOTEwNjAzMl5BMl5BanBnXkFtZTcwODc5MTUwMw@@._V1_SX300.jpg",
"Metascore": "83",
"imdbRating": "7.9",
"imdbVotes": "876,575",
"imdbID": "tt0499549",
"Type": "movie",
"Response": "True"
}
];
// Add your code below this line
const averageRating = watchList.filter(film => film.Director === 'Christopher Nolan').map(film => Number(film.imdbRating)).reduce((total, film) => total + film) / watchList.filter(film => film.Director === 'Christopher Nolan').length;
// Add your code above this line
console.log(averageRating);
The sort method sorts the elements of an array according to the callback function.
For example:
function ascendingOrder(arr) {
return arr.sort(function(a, b) {
return a - b;
});
}
ascendingOrder([1, 5, 2, 3, 4]);
// Returns [1, 2, 3, 4, 5]
function reverseAlpha(arr) {
return arr.sort(function(a, b) {
return a < b;
});
}
reverseAlpha(['l', 'h', 'z', 'b', 's']);
// Returns ['z', 's', 'l', 'h', 'b']
Note: It's encouraged to provide a callback function to specify how to sort the array items. JavaScript's default sorting method is by string Unicode point value, which may return unexpected results.
Use the sort method in the alphabeticalOrder function to sort the elements of arr in alphabetical order.
function alphabeticalOrder(arr) {
// Add your code below this line
return arr.sort(function(a,b) {
return a > b;
});
// Add your code above this line
}
alphabeticalOrder(["a", "d", "c", "a", "z", "g"]);
A side effect of the sort method is that it changes the order of the elements in the original array. In other words, it mutates the array in place. One way to avoid this is to first concatenate an empty array to the one being sorted (remember that concat returns a new array), then run the sort method.
Use the sort method in the nonMutatingSort function to sort the elements of an array in ascending order. The function should return a new array, and not mutate the globalArray variable.
var globalArray = [5, 6, 3, 2, 9];
function nonMutatingSort(arr) {
// Add your code below this line
return [].concat(arr).sort((a, b) => a - b);
// Add your code above this line
}
nonMutatingSort(globalArray);
The split method splits a string into an array of strings. It takes an argument for the delimiter, which can be a character to use to break up the string or a regular expression. For example, if the delimiter is a space, you get an array of words, and if the delimiter is an empty string, you get an array of each character in the string.
Here are two examples that split one string by spaces, then another by digits using a regular expression:
var str = "Hello World";
var bySpace = str.split(" ");
// Sets bySpace to ["Hello", "World"]
var otherString = "How9are7you2today";
var byDigits = otherString.split(/\d/);
// Sets byDigits to ["How", "are", "you", "today"]
Since strings are immutable, the split method makes it easier to work with them.
Use the split method inside the splitify function to split str into an array of words. The function should return the array. Note that the words are not always separated by spaces, and the array should not contain punctuation.
function splitify(str) {
// Add your code below this line
return str.split(/\W/)
// Add your code above this line
}
splitify("Hello World,I-am code");
The join method is used to join the elements of an array together to create a string. It takes an argument for the delimiter that is used to separate the array elements in the string.
Here's an example:
var arr = ["Hello", "World"];
var str = arr.join(" ");
// Sets str to "Hello World"
Use the join method (among others) inside the sentensify function to make a sentence from the words in the string str. The function should return a string. For example, "I-like-Star-Wars" would be converted to "I like Star Wars". For this challenge, do not use the replace method.
function sentensify(str) {
// Add your code below this line
return str.split(/\W/).join(' ');
// Add your code above this line
}
sentensify("May-the-force-be-with-you");
The last several challenges covered a number of useful array and string methods that follow functional programming principles. We've also learned about reduce, which is a powerful method used to reduce problems to simpler forms. From computing averages to sorting, any array operation can be achieved by applying it. Recall that map and filter are special cases of reduce.
Let's combine what we've learned to solve a practical problem.
Many content management sites (CMS) have the titles of a post added to part of the URL for simple bookmarking purposes. For example, if you write a Medium post titled "Stop Using Reduce", it's likely the URL would have some form of the title string in it (".../stop-using-reduce"). You may have already noticed this on the freeCodeCamp site.
Fill in the urlSlug function so it converts a string title and returns the hyphenated version for the URL. You can use any of the methods covered in this section, and don't use replace. Here are the requirements:
The input is a string with spaces and title-cased words
The output is a string with the spaces between words replaced by a hyphen (-)
The output should be all lower-cased letters
The output should not have any spaces.
// the global variable
var globalTitle = "Winter Is Coming";
// Add your code below this line
function urlSlug(title) {
return title.toLowerCase().trim().split(/\s+/).join('-');
}
// Add your code above this line
var winterComing = urlSlug(globalTitle); // Should be "winter-is-coming"
The every method works with arrays to check if every element passes a particular test. It returns a Boolean value - true if all values meet the criteria, false if not.
For example, the following code would check if every element in the numbers array is less than 10:
var numbers = [1, 5, 8, 0, 10, 11];
numbers.every(function(currentValue) {
return currentValue < 10;
});
// Returns false
Use the every method inside the checkPositive function to check if every element in arr is positive. The function should return a Boolean value.
function checkPositive(arr) {
// Add your code below this line
return arr.every(el => el > 0);
// Add your code above this line
}
checkPositive([1, 2, 3, -4, 5]);
The some method works with arrays to check if any element passes a particular test. It returns a Boolean value - true if any of the values meet the criteria, false if not.
For example, the following code would check if any element in the numbers array is less than 10:
var numbers = [10, 50, 8, 220, 110, 11];
numbers.some(function(currentValue) {
return currentValue < 10;
});
// Returns true
Use the some method inside the checkPositive function to check if any element in arr is positive. The function should return a Boolean value.
function checkPositive(arr) {
// Add your code below this line
return arr.some(el => el > 0);
// Add your code above this line
}
checkPositive([1, 2, 3, -4, 5]);
The arity of a function is the number of arguments it requires. Currying a function means to convert a function of N arity into N functions of arity 1.
In other words, it restructures a function so it takes one argument, then returns another function that takes the next argument, and so on.
Here's an example:
//Un-curried function
function unCurried(x, y) {
return x + y;
}
//Curried function
function curried(x) {
return function(y) {
return x + y;
}
}
curried(1)(2) // Returns 3
This is useful in your program if you can't supply all the arguments to a function at one time. You can save each function call into a variable, which will hold the returned function reference that takes the next argument when it's available. Here's an example using the curried function in the example above:
// Call a curried function in parts:
var funcForY = curried(1);
console.log(funcForY(2)); // Prints 3
Similarly, partial application can be described as applying a few arguments to a function at a time and returning another function that is applied to more arguments.
Here's an example:
//Impartial function
function impartial(x, y, z) {
return x + y + z;
}
var partialFn = impartial.bind(this, 1, 2);
partialFn(10); // Returns 13
Fill in the body of the add function so it uses currying to add parameters x, y, and z.
function add(x) {
// Add your code below this line
return function(y) {
return function(z) {
return x + y + z;
}
}
// Add your code above this line
}
add(10)(20)(30);