Array.prototype.length - JavaScript 编辑

The length property of an object which is an instance of type Array sets or returns the number of elements in that array. The value is an unsigned, 32-bit integer that is always numerically greater than the highest index in the array.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Description

The value of the length property is an integer with a positive sign and a value less than 2 to the 32nd power (2³²).

var namelistA = new Array(4294967296); //2 to the 32nd power = 4294967296
var namelistC = new Array(-100) //negative sign

console.log(namelistA.length); //RangeError: Invalid array length
console.log(namelistC.length); //RangeError: Invalid array length

var namelistB = [];
namelistB.length = Math.pow(2,32)-1; //set array length less than 2 to the 32nd power
console.log(namelistB.length);

//4294967295

You can set the length property to truncate an array at any time. When you extend an array by changing its length property, the number of actual elements increases; for example, if you set length to 3 when it is currently 2, the array now contains 3 elements, which causes the third element to be a non-iterable empty slot.

const arr = [1, 2];
console.log(arr);
// [ 1, 2 ]

arr.length = 5; // set array length to 5 while currently 2.
console.log(arr);
// [ 1, 2, <3 empty items> ]

arr.forEach(element => console.log(element));
// 1
// 2

As you can see, the length property does not necessarily indicate the number of defined values in the array. See also Relationship between length and numerical properties.

Property attributes of Array.prototype.length
Writable	yes
Enumerable	no
Configurable	no

• Writable: If this attribute set to false, the value of the property cannot be changed.
• Configurable: If this attribute set to false, any attempts to delete the property or change its attributes (Writable, Configurable, or Enumerable) will fail.
• Enumerable: If this attribute set to true, the property will be iterated over during for or for..in loops.

Examples

Iterating over an array

In the following example, the array numbers is iterated through by looking at the length property. The value in each element is then doubled.

var numbers = [1, 2, 3, 4, 5];
var length = numbers.length;
for (var i = 0; i < length; i++) {
  numbers[i] *= 2;
}
// numbers is now [2, 4, 6, 8, 10]

Shortening an array

The following example shortens the array numbers to a length of 3 if the current length is greater than 3.

var numbers = [1, 2, 3, 4, 5];

if (numbers.length > 3) {
  numbers.length = 3;
}

console.log(numbers); // [1, 2, 3]
console.log(numbers.length); // 3

Create empty array of fixed length

var numbers = [];
numbers.length = 3;
console.log(numbers); // [undefined, undefined, undefined]

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'Array.length' in that specification.

Browser compatibility

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The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

See also

• Array