add fromModernJulianDate()

.npmignore

@@ -12,6 +12,7 @@ _config.yml
 .gitignore
 .npmignore
 .prettierignore
-.prettierrc.json
+
 eslint.config.*
+prettier.config.*
 tsconfig.json

README.md

@@ -18,13 +18,14 @@ npm install @cityssm/modern-julian-date
 
 ```javascript
 import {
+  fromModernJulianDate,
   toModernJulianDate,
   toShortModernJulianDate
 } from "@cityssm/modern-julian-date";
 
 /*
  * toModernJulianDate()
- * returns date strings in YYYYDDD format.
+ * Returns date strings in YYYYDDD format.
  */
 
 console.log( toModernJulianDate(new Date(2022, 1 - 1, 1)) );
@@ -38,9 +39,17 @@ console.log( toModernJulianDate(new Date(2020, 12 - 1, 31)) );
 
 /*
  * toShortModernJulianDate()
- * returns date strings in YYDDD format.
+ * Returns date strings in YYDDD format.
  */
 
 console.log( toShortModernJulianDate(new Date(2022, 1 - 1, 1)) );
 // => '22001'
+
+/*
+ * fromModernJulianDate()
+ * Returns Date objects.
+ */
+
+console.log( fromModernJulianDate('2020366') )
+// => 'Thu Dec 31 2020 00:00:00 GMT-0500 (Eastern Standard Time)'
 ```

index.d.ts

@@ -1,4 +1,5 @@
 export declare function getDayOfYear(date: Date, startAtZero?: boolean): number;
 export declare function toModernJulianDate(date: Date): string;
 export declare function toShortModernJulianDate(date: Date): string;
+export declare function fromModernJulianDate(modernJulianDate: string): Date;
 export default toModernJulianDate;

index.js

@@ -12,4 +12,22 @@ export function toModernJulianDate(date) {
 export function toShortModernJulianDate(date) {
     return toModernJulianDate(date).slice(2);
 }
+export function fromModernJulianDate(modernJulianDate) {
+    let sevenDigitModernJulianDate = modernJulianDate;
+    if (modernJulianDate.length === 5) {
+        sevenDigitModernJulianDate =
+            Math.floor(new Date().getFullYear() / 100).toString() + modernJulianDate;
+    }
+    if (sevenDigitModernJulianDate.length !== 7) {
+        throw new Error(`Invalid modern Julian date: ${modernJulianDate}`);
+    }
+    const year = Number.parseInt(sevenDigitModernJulianDate.slice(0, 4));
+    const days = Number.parseInt(sevenDigitModernJulianDate.slice(4));
+    if (Number.isNaN(year) || Number.isNaN(days) || days > 366) {
+        throw new TypeError(`Invalid modern Julian date: ${modernJulianDate}`);
+    }
+    const date = new Date(year, 1 - 1, 1);
+    date.setDate(days);
+    return date;
+}
 export default toModernJulianDate;

index.ts

@@ -37,4 +37,37 @@ export function toShortModernJulianDate(date: Date): string {
   return toModernJulianDate(date).slice(2)
 }
 
+/**
+ * Converts a modern Julian date into a Javascript Date object.
+ * ex. '2023001' => 2023-01-01
+ * @param modernJulianDate - A five-digit (short) or seven-digit modern Julian date. Note that five-digit dates will be assumed to be in the current century.
+ * @returns Date object.
+ */
+export function fromModernJulianDate(modernJulianDate: string): Date {
+  let sevenDigitModernJulianDate = modernJulianDate
+
+  if (modernJulianDate.length === 5) {
+    sevenDigitModernJulianDate =
+      Math.floor(new Date().getFullYear() / 100).toString() + modernJulianDate
+  }
+
+  if (sevenDigitModernJulianDate.length !== 7) {
+    throw new Error(`Invalid modern Julian date: ${modernJulianDate}`)
+  }
+
+  const year = Number.parseInt(sevenDigitModernJulianDate.slice(0, 4))
+  const days = Number.parseInt(sevenDigitModernJulianDate.slice(4))
+
+  if (Number.isNaN(year) || Number.isNaN(days) || days > 366) {
+    throw new TypeError(`Invalid modern Julian date: ${modernJulianDate}`)
+  }
+
+  // eslint-disable-next-line sonarjs/no-identical-expressions
+  const date = new Date(year, 1 - 1, 1)
+
+  date.setDate(days)
+
+  return date
+}
+
 export default toModernJulianDate