small refactor to substr function to ease with porting

lib/standard-clojure-style.js

@@ -184,14 +184,13 @@
     return s.charAt(n)
   }
 
-  // Returns the substring of s beginning at start inclusive, and ending
-  // at end (defaults to length of string), exclusive.
-  function substr (s, start, end) {
+  // Returns the substring of s beginning at startIdx inclusive, and ending
+  // at endIdx, exclusive.
+  // Pass -1 to endIdx to mean "until the end of the string"
+  function substr (s, startIdx, endIdx) {
     const len = strLen(s)
-    if (!isPositiveInt(end)) end = len
-    if (end > len) end = len
-    // TODO: throw here if end < start?
-    return s.substring(start, end)
+    if (endIdx < 0) endIdx = len
+    return s.substring(startIdx, endIdx)
   }
 
   function repeatString (text, n) {
@@ -399,7 +398,7 @@
       pattern_str: opts.regex,
       parse: (txt, pos) => {
         // NOTE: this might be a perf issue; investigate later
-        const txt2 = substr(txt, pos)
+        const txt2 = substr(txt, pos, -1)
         const result = txt2.match(opts.regex)
 
         // HACK HACK HACK:
@@ -2335,7 +2334,7 @@
       } else if (insideGenClass && idx > genClassNodeIdx && isTextNode && genClassToggle === 0 && isGenClassKeyword(node)) {
         result.genClass.isEmpty = false
 
-        genClassKeyStr = substr(node.text, 1)
+        genClassKeyStr = substr(node.text, 1, -1)
         result.genClass[genClassKeyStr] = {}
 
         // add commentsAbove if possible

test/internals.test.js

@@ -22,7 +22,7 @@ test('String util: substr', () => {
   expect(scsLib._substr('abcdef', 0, 0)).toBe('')
   expect(scsLib._substr('abcdef', 0, 2)).toBe('ab')
   expect(scsLib._substr('abcdef', 3, 5)).toBe('de')
-  expect(scsLib._substr('abcdef', 2)).toBe('cdef')
+  expect(scsLib._substr('abcdef', 2, -1)).toBe('cdef')
 })
 
 test('commentNeedsSpaceBefore', () => {