arguments
是
Array
-like object accessible inside
functions
that contains the values of the arguments passed to that function.
注意： If you're writing ES6 compatible code, then rest parameters should be preferred.
注意：
“Array-like” means that
arguments
has a
length
property and properties indexed from zero, but it doesn't have
Array
's built-in methods like
forEach()
or
map()
。见
§Description
了解细节。
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';
arguments
object is not an
Array
. It is similar, but lacks all
Array
properties except
length
. For example, it does not have the
pop()
方法。
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];
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;
}
可以使用
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
特性。
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]
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>"
*/
arguments
object can be used in conjunction with
rest
,
default
，和
destructured
参数。
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.
注意：
You cannot write a
"use strict";
directive in the body of a function definition that accepts rest, default, or destructured parameters. Doing so will throw
a syntax error
.
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
| 规范 |
|---|
|
ECMAScript (ECMA-262)
The definition of 'Arguments Exotic Objects' in that specification. |
| Desktop | Mobile | Server | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
arguments
|
Chrome 1 | Edge 12 | Firefox 1 | IE 3 | Opera 3 | Safari 1 | WebView Android 1 | Chrome Android 18 | Firefox Android 4 | Opera Android 10.1 | Safari iOS 1 | Samsung Internet Android 1.0 | nodejs Yes |
callee
|
Chrome 1 | Edge 12 | Firefox 1 | IE 6 | Opera 4 | Safari 1 | WebView Android 1 | Chrome Android 18 | Firefox Android 4 | Opera Android 10.1 | Safari iOS 1 | Samsung Internet Android 1.0 | nodejs Yes |
length
|
Chrome 1 | Edge 12 | Firefox 1 | IE 4 | Opera 4 | Safari 1 | WebView Android 1 | Chrome Android 18 | Firefox Android 4 | Opera Android 10.1 | Safari iOS 1 | Samsung Internet Android 1.0 | nodejs Yes |
@@iterator
|
Chrome 52 | Edge 12 | Firefox 46 | IE No | Opera 39 | Safari 9 | WebView Android 52 | Chrome Android 52 | Firefox Android 46 | Opera Android 41 | Safari iOS 9 | Samsung Internet Android 6.0 | nodejs Yes |
完整支持
不支持