HTML String replace that maintains the original whitespace indentation of the token in the template.

replaceMaintainingTokenIndentation.js

  /* 
   * This function replaces the token string with the replacement string, but maintains
   * the original whitespace indentation the token had within the template.
   *
   * For example if your template has:
   *
   * <div>
   *   {% TOKEN %}
   * </div>
   *
   * Then the output would be:
   *
   * <div>
   *   Hello
   *   World
   * </div>
   * 
   */
  replaceMaintainingTokenIndentation: function( string, token, replacement ){
    // just quickly remove the token if there is nothing to replace it with
    if ( replacement === undefined || replacement == '' ){
      return string.replace( new RegExp( token, 'g' ), '' );
    }

    var index = string.indexOf( token ) - 1;
    
    // replace each occurrence of the token

    while ( index >= 0 ){
      var indent = '';

      // collect the whitespace between the token and the previous non-whitespace character
      for ( index; index >= 0; index-- ){
        var character = string.charAt( index );

        // stop if we encounter non-whitespace
        if ( character != ' ' && character != '\t' )
          break;

        indent += character;
      }

      // indent all but the first line of the replacement string
      var newReplacement = replacement.replace( /\n/g, '\n' + indent );  

      // replace this occurrence of token with the properly indented replacement
      string = string.replace( token, newReplacement );

      // move on to the next occurence of the token
      index = string.indexOf( token ) - 1;
    }

    return string;

  }