fix!: correct index value in callback (BREAKING CHANGE)

The index parameter passed to the replacement callback now returns
logical match index (0, 1, 2, 3...) instead of internal array index
(1, 3, 5, 7...).

This is more intuitive and useful for React key props.

BREAKING CHANGE: The second parameter to the replacement callback
now starts at 0 and increments by 1, rather than starting at 1 and
incrementing by 2.

Fixes #32
Closes #92

Author: iansinnott

index.js

@@ -83,7 +83,7 @@ function replaceString(str, match, fn, count = null) {
 
     curCharLen = result[i].length;
     curCharStart += result[i - 1].length;
-    result[i] = fn(result[i], i, curCharStart);
+    result[i] = fn(result[i], (i - 1) / 2, curCharStart);
     curCharStart += curCharLen;
   }
 

test.js

@@ -172,9 +172,9 @@ test("Avoids undefined values due to regex", (t) => {
   });
 });
 
-test("Fixed number of string replacements", (t) => { 
+test("Fixed number of string replacements", (t) => {
   const string = `Test hey test hey test`;
-  const replacedContent = reactStringReplace(string, 'hey', match => { 
+  const replacedContent = reactStringReplace(string, 'hey', match => {
     return 'lo';
   },1);
   t.deepEqual(replacedContent, [
@@ -184,4 +184,15 @@ test("Fixed number of string replacements", (t) => {
     'hey',
     ' test'
   ])
-})
+});
+
+test("Indexes start at 0 and are contiguous", t => {
+  const string = 'Hello there general Kenobi';
+  const re = /(\w+)/;
+
+  let expectedIndex = 0;
+  replaceString(string, re, (match, index) => {
+    t.deepEqual(expectedIndex, index);
+    expectedIndex++;
+  });
+});