Array.prototype.shift() - JavaScript 编辑

The shift() method removes the first element from an array and returns that removed element. This method changes the length of the array.

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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.

Syntax

arr.shift()

Return value

The removed element from the array; undefined if the array is empty.

Description

The shift method removes the element at the zeroeth index and shifts the values at consecutive indexes down, then returns the removed value. If the length property is 0, undefined is returned.

shift 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.

Array.prototype.pop() has similar behavior to shift, but applied to the last element in an array.

Examples

Removing an element from an array

The following code displays the myFish array before and after removing its first element. It also displays the removed element:

var myFish = ['angel', 'clown', 'mandarin', 'surgeon'];

console.log('myFish before:', JSON.stringify(myFish));
// myFish before: ['angel', 'clown', 'mandarin', 'surgeon']

var shifted = myFish.shift();

console.log('myFish after:', myFish);
// myFish after: ['clown', 'mandarin', 'surgeon']

console.log('Removed this element:', shifted);
// Removed this element: angel

Using shift() method in while loop

The shift() method is often used in condition inside while loop. In the following example every iteration will remove the next element from an array, until it is empty:

var names = ["Andrew", "Edward", "Paul", "Chris" ,"John"];

while( (i = names.shift()) !== undefined ) {
    console.log(i);
}
// Andrew, Edward, Paul, Chris, John

Specifications

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

Browser compatibility

BCD tables only load in the browser

See also

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