O objeto arguments

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

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Sintaxe

arguments

Descrição

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 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);
// 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 operator to convert arguments to a real Array:

var args = Array.from(arguments);
var 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.

Using typeof with Arguments

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

Propriedades

arguments.callee

Reference to the currently executing function that the arguments belong to.

arguments.caller

Reference to the function that invoked the currently executing function.

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 the arguments.

Exemplos

Defining a function that concatenates several strings

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) {
  var 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');

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 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:

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

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

Rest, default, and destructured parameters

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 is a subtle difference for non-strict code.

If a non-strict function does not contain rest, default, or destructured parameters, then the values in the arguments object do change in sync with the values of the argument variables. See the code below:

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

and

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

When a non-strict function does contain rest, default, or destructured parameters, then the values in the arguments object do not track the values of the arguments. Instead, they reflect the arguments provided when the function was called:

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

and

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

and

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

Especificações

Compatibildiade de navegador

Consultar também

• Function
• Rest parameters