Added insert method for inserting substrings at a specific index

dist/string.js

@@ -276,6 +276,17 @@ string.js - Copyright (C) 2012-2014, JP Richardson <jprichardson@gmail.com>
       return new this.constructor(s)
     },
 
+    insert: function(ss, index) {
+      if (this.s === null || this.s === undefined)
+        return new this.constructor('')
+
+      ss = ss || ''
+      index = parseInt(index, 10) || 0
+
+      var s = this.s.slice(0, index) + ss + this.s.slice(index)
+      return new this.constructor(s)
+    },
+
     isAlpha: function() {
       return !/[^a-z\xDF-\xFF]|^$/.test(this.s.toLowerCase());
     },

lib/string.js

@@ -199,6 +199,17 @@ string.js - Copyright (C) 2012-2014, JP Richardson <jprichardson@gmail.com>
       return new this.constructor(s)
     },
 
+    insert: function(ss, index) {
+      if (this.s === null || this.s === undefined)
+        return new this.constructor('')
+
+      ss = ss || ''
+      index = parseInt(index, 10) || 0
+
+      var s = this.s.slice(0, index) + ss + this.s.slice(index)
+      return new this.constructor(s)
+    },
+
     isAlpha: function() {
       return !/[^a-z\xDF-\xFF]|^$/.test(this.s.toLowerCase());
     },

test/string.test.js

@@ -207,6 +207,14 @@
       })
     })
 
+    describe('- insert(substring, index)', function() {
+      it('should insert `substring` at the specified index in the original string', function() {
+        EQ (S('the string').insert('prefix ').s, 'prefix the string')
+        EQ (S('we can do this').insert(' too', 14).s, 'we can do this too')
+        EQ (S('this is cool').insert('un', 8).s, 'this is uncool')
+      })
+    })
+
     describe('- isAlpha()', function() {
       it("should return true if the string contains only letters", function() {
         T (S("afaf").isAlpha());