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    Arguments object

    This article is in need of a technical review.

    Summary

    The arguments object is an Array-like object corresponding to the arguments passed to a function.

    Syntax

    arguments

    Description

    The arguments object is a local variable available within all functions; arguments as a property of Function can no longer be used.

    You can refer to a function's arguments within the function by using the arguments object. This object contains an entry for each argument passed to the function, the first entry's index starting at 0. For example, if a function is passed three arguments, you can refer to the argument as follows:

    arguments[0]
    arguments[1]
    arguments[2]
    

    The arguments can also be set:

    arguments[1] = 'new value';

    The arguments object is not an Array. It is similar to an Array, but does not have any Array properties except length. For example, it does not have the pop method. However it can be converted to a real Array:

    var args = Array.prototype.slice.call(arguments);

    You should not slice on arguments because it prevents optimizations in JavaScript engines (V8 for example). Instead, try constructing a new array by iterating through the arguments object.

    If Array generics are available, one can use the following instead:

    var args = Array.slice(arguments);

    The arguments object is available only within a function body. Attempting to access the arguments object outside a function declaration results in an error.

    You can use the arguments object if you call a function with more arguments than it is formally declared to accept. This technique is useful for functions that can be passed a variable number of arguments. You can use arguments.length to determine the number of arguments passed to the function, and then process each argument by using the arguments object. (To determine the number of arguments declared when a function was defined, use the Function.length property.)

    Properties

    arguments.callee
    Reference to the currently executing function.
    arguments.caller
    Reference to the function that invoked the currently executing function.
    arguments.length
    Reference to the number of arguments passed to the function.

    Examples

    Example: Defining a function that concatenates several strings

    This example defines a function that concatenates several strings. The only formal argument for the function is a string that specifies the characters that separate the items to concatenate. The function is defined as follows:

    function myConcat(separator) {
      var args = Array.prototype.slice.call(arguments, 1);
      return args.join(separator);
    }

    You can pass any number of arguments to this function, and it creates a list using each argument as an item in the list.

    // returns "red, orange, blue"
    myConcat(", ", "red", "orange", "blue");
    
    // returns "elephant; giraffe; lion; cheetah"
    myConcat("; ", "elephant", "giraffe", "lion", "cheetah");
    
    // returns "sage. basil. oregano. pepper. parsley"
    myConcat(". ", "sage", "basil", "oregano", "pepper", "parsley");

    Example: Defining a function that creates HTML lists

    This example defines a function that creates a string containing HTML for a list. The only formal argument for the function is a string that is "u" if the list is to be unordered (bulleted), or "o" if the list is to be ordered (numbered). The function is defined as follows:

    function list(type) {
      var result = "<" + type + "l><li>";
      var args = Array.prototype.slice.call(arguments, 1);
      result += args.join("</li><li>");
      result += "</li></" + type + "l>"; // end list
    
      return result;
    }

    You can pass any number of arguments to this function, and it adds each argument as an item to a list of the type indicated. For example:

    var listHTML = list("u", "One", "Two", "Three");
    
    /* listHTML is:
    
    "<ul><li>One</li><li>Two</li><li>Three</li></ul>"
    
    */

    Specifications

    Specification Status Comment
    ECMAScript 1st Edition. Standard Initial definition. Implemented in JavaScript 1.1
    ECMAScript 5.1 (ECMA-262)
    The definition of 'Arguments Object' in that specification.
    Standard  
    ECMAScript 6 (ECMA-262)
    The definition of 'Arguments Exotic Objects' in that specification.
    Release Candidate  

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

    See also