Array.prototype.unshift() - JavaScript 编辑

The unshift() method adds one or more elements to the beginning of an array and returns the new length of the array.

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Syntax

arr.unshift(element1[, ...[, elementN]])

Parameters

elementN

The elements to add to the front of the arr.

Return value

The new length property of the object upon which the method was called.

Description

The unshift method inserts the given values to the beginning of an array-like object.

unshift is intentionally generic. This method can be called or applied to objects resembling arrays. Objects which do not contain a length property—reflecting the last in a series of consecutive, zero-based numerical properties—may not behave in any meaningful manner.

Please note that, if multiple elements are passed as parameters, they're inserted in chunk at the beginning of the object, in the exact same order they were passed as parameters. Hence, calling unshift with n arguments once, or calling it n times with 1 argument (with a loop, for example), don't yield the same results.

See example:

let arr = [4, 5, 6]

arr.unshift(1, 2, 3)
console.log(arr);
// [1, 2, 3, 4, 5, 6]

arr = [4, 5, 6] // resetting the array

arr.unshift(1)
arr.unshift(2)
arr.unshift(3)

console.log(arr)
// [3, 2, 1, 4, 5, 6]

Examples

Using unshift

let arr = [1, 2]

arr.unshift(0)               // result of the call is 3, which is the new array length
// arr is [0, 1, 2]

arr.unshift(-2, -1)          // the new array length is 5
// arr is [-2, -1, 0, 1, 2]

arr.unshift([-4, -3])        // the new array length is 6
// arr is [[-4, -3], -2, -1, 0, 1, 2]

arr.unshift([-7, -6], [-5])  // the new array length is 8
// arr is [ [-7, -6], [-5], [-4, -3], -2, -1, 0, 1, 2 ]

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Array.prototype.push()
• Array.prototype.pop()
• Array.prototype.shift()
• Array.prototype.concat()