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function expression

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Từ khóa function (hàm) có thể dùng để định nghĩa chức năng bên trong một biểu thức.

Cú pháp

var myFunction = function [name]([param1[, param2[, ..., paramN]]]) {
   statements
};

Tham số

name
Tên hàm. Có thể bỏ qua, trong trường hợp chức năng đó là vô danh. Nó mô tả chính xác nhiệm vụ mà hàm sẽ làm.
paramN
Tên các đối số truyền vào.
statements
Các câu lệnh xử lý các đối số paramN truyền vào.

Mô tả

Một biểu thức chức năng (function) is very similar to and has almost the same syntax as a function statement (see function statement for details). The main difference between a function expression and a function statement is the function name, which can be omitted in function expressions to create anonymous functions. A function expression can be used as a IIFE (Immediately Invoked Function Expression) which runs as soon as it is defined. See also the chapter about functions for more information.

Function expression hoisting

Function expressions in JavaScript are not hoisted, unlike function declarations. You can't use function expressions before you define them:

notHoisted(); // TypeError: notHoisted is not a function

var notHoisted = function() {
   console.log('bar');
};

Examples

The following example defines an unnamed function and assigns it to x. The function returns the square of its argument:

var x = function(y) {
   return y * y;
};

Named function expression

If you want to refer to the current function inside the function body, you need to create a named function expression. This name is then local only to the function body (scope). This also avoids using the non-standard arguments.callee property.

var math = {
  'factorial': function factorial(n) {
    if (n <= 1)
      return 1;
    return n * factorial(n - 1);
  }
};

Specifications

Specification Status Comment
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Function definitions' in that specification.
Draft  
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Function definitions' in that specification.
Standard  
ECMAScript 5.1 (ECMA-262)
The definition of 'Function definition' in that specification.
Standard  
ECMAScript 3rd Edition (ECMA-262)
The definition of 'Function definition' in that specification.
Standard Initial definition. Implemented in JavaScript 1.5.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
Trailing comma in parameters ? 52.0 (52.0) ? ? ?
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
Trailing comma in parameters ? ? 52.0 (52.0) ? ? ? ?

See also

Document Tags and Contributors

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 Last updated by: anhnc,