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# Javascript Array reduce() Method

## Description:

Javascript array reduce() method applies a function simultaneously against two values of the array (from left-to-right) as to reduce it to a single value.

## Syntax:

    array.reduce(callback[, initialValue]);

Here is the detail of parameters:

- callback : Function to execute on each value in the array.

- initialValue : Object to use as the first argument to the first call of the callback.

## Return Value:

Returns the reduced single value of the array.

## Compatibility:

This method is a JavaScript extension to the ECMA-262 standard; as such it may not be present in other implementations of the standard. To make it work you need to add following code at the top of your script:

    if (!Array.prototype.reduce)
    {
      Array.prototype.reduce = function(fun /*, initial*/)
      {
        var len = this.length;
        if (typeof fun != "function")
          throw new TypeError();

        // no value to return if no initial value and an empty array
        if (len == 0 && arguments.length == 1)
          throw new TypeError();

        var i = 0;
        if (arguments.length >= 2)
        {
          var rv = arguments[1];
        }
        else
        {
          do
          {
            if (i in this)
            {
              rv = this[i++];
              break;
            }

            // if array contains no values, no initial value to return
            if (++i >= len)
              throw new TypeError();
          }
          while (true);
        }

        for (; i < len; i++)
        {
          if (i in this)
            rv = fun.call(null, rv, this[i], i, this);
        }

        return rv;
      };
    }

## Example:

    <html>
    <head>
    <title>JavaScript Array reduce Method</title>
    </head>
    <body>
    <script type="text/javascript">
    if (!Array.prototype.reduce)
    {
      Array.prototype.reduce = function(fun /*, initial*/)
      {
        var len = this.length;
        if (typeof fun != "function")
          throw new TypeError();

        // no value to return if no initial value and an empty array
        if (len == 0 && arguments.length == 1)
          throw new TypeError();

        var i = 0;
        if (arguments.length >= 2)
        {
          var rv = arguments[1];
        }
        else
        {
          do
          {
            if (i in this)
            {
              rv = this[i++];
              break;
            }

            // if array contains no values, no initial value to return
            if (++i >= len)
              throw new TypeError();
          }
          while (true);
        }

        for (; i < len; i++)
        {
          if (i in this)
            rv = fun.call(null, rv, this[i], i, this);
        }

        return rv;
      };
    }

    var total = [0, 1, 2, 3].reduce(function(a, b){ return a + b; });
    document.write("total is : " + total );
    </script>
    </body>
    </html>

This will produce following result:

    total is : 6

To understand it in better way you can Try it yourself.

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