Insert a string at a specific index

Question

How can I insert a string at a specific index of another string?

 var txt1 = \"foo baz\"

Suppose I want to insert \"bar \" after the \"foo

Answer 1

This is basically doing what @Base33 is doing except I'm also giving the option of using a negative index to count from the end. Kind of like the substr method allows.

// use a negative index to insert relative to the end of the string.

String.prototype.insert = function (index, string) {
  var ind = index < 0 ? this.length + index  :  index;
  return  this.substring(0, ind) + string + this.substr(ind);
};

Example: Let's say you have full size images using a naming convention but can't update the data to also provide thumbnail urls.

var url = '/images/myimage.jpg';
var thumb = url.insert(-4, '_thm');

//    result:  '/images/myimage_thm.jpg'