The arguments object

arguments is an Array-like object accessible inside functions that contains the values of the arguments passed to that function.

The arguments object is a local variable available within all non-arrow functions. You can refer to a function's arguments inside that function by using its arguments object. It has entries for each argument the function was called with, with the first entry's index at 0.

For example, if a function is passed 3 arguments, you can access them as follows:

arguments[0] // first argument
arguments[1] // second argument
arguments[2] // third argument

Each argument can also be set or reassigned:

arguments[1] = 'new value';

The arguments object is not an Array. It is similar, but lacks all 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);
// Using an array literal is shorter than above but allocates an empty array
var args = [].slice.call(arguments);

As you can do with any Array-like object, you can use ES2015's Array.from() method or spread syntax to convert arguments to a real Array:

let args = Array.from(arguments);
// or
let args = [...arguments];

The arguments object is useful for functions called with more arguments than they are formally declared to accept. This technique is useful for functions that can be passed a variable number of arguments, such as Math.min(). This example function accepts any number of string arguments and returns the longest one:

function longestString() {
  var longest = '';
  for (var i=0; i < arguments.length; i++) {
    if (arguments[i].length > longest.length) {
      longest = arguments[i];
    }
  }
  return longest;
}

You can use arguments.length to count how many arguments the function was called with. If you instead want to count how many parameters a function is declared to accept, inspect that function's length property.

The typeof operator returns 'object' when used with arguments

console.log(typeof arguments); // 'object' 

The type of individual arguments can be determined by indexing arguments:

console.log(typeof arguments[0]); // returns the type of the first argument

arguments.callee

Reference to the currently executing function that the arguments belong to. Forbidden in strict mode.

arguments.length

The number of arguments that were passed to the function.

arguments[@@iterator]

Returns a new Array iterator object that contains the values for each index in arguments.

This example defines a function that concatenates several strings. The function's only formal argument is a string containing the characters that separate the items to concatenate.

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

You can pass as many arguments as you like to this function. It returns a string list using each argument 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');

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

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

let listHTML = list('u', 'One', 'Two', 'Three');

/* listHTML is:
"<ul><li>One</li><li>Two</li><li>Three</li></ul>"
*/

The arguments object can be used in conjunction with rest, default, and destructured parameters.

function foo(...args) {
  return args;
}
foo(1, 2, 3); // [1, 2, 3]

While the presence of rest, default, or destructured parameters does not alter the behavior of the arguments object in strict mode code, there are subtle differences for non-strict code.

In strict-mode code, the arguments object behaves the same whether or not a function is passed rest, default, or destructured parameters. That is, assigning new values to variables in the body of the function will not affect the arguments object. Nor will assigning new variables to the arguments object affect the value of variables.

Non-strict functions that are passed only simple parameters (that is, not rest, default, or restructured parameters) will sync the value of variables new values in the body of the function with the arguments object, and vice versa:

function func(a) {
  arguments[0] = 99; // updating arguments[0] also updates a
  console.log(a);
}
func(10); // 99

And also:

function func(a) {
  a = 99; // updating a also updates arguments[0]
  console.log(arguments[0]);
}
func(10); // 99

Conversely, non-strict functions that are passed rest, default, or destructured parameters will not sync new values assigned to argument variables in the function body with the arguments object. Instead, the arguments object in non-strict functions with complex parameters will always reflect the values passed to the function when the function was called (this is the same behavior as exhibited by all strict-mode functions, regardless of the type of variables they are passed):

function func(a = 55) {
  arguments[0] = 99; // updating arguments[0] does not also update a
  console.log(a);
}
func(10); // 10

And also:

function func(a = 55) {
  a = 99; // updating a does not also update arguments[0]
  console.log(arguments[0]);
}
func(10); // 10

And also:

// An untracked default parameter
function func(a = 55) {
  console.log(arguments[0]);
}
func(); // undefined

• Function
• Rest parameters