The shift()
method of Array instances removes the first
element from an array and returns that removed element. This method changes the length
of the array.
Syntax
shift()
Parameters
None.
Return value
The removed element from the array; undefined if the array is empty.
Description
The shift()
method removes the element at the zeroth index and shifts the
values at consecutive indexes down, then returns the removed value. If the
length property is 0, undefined is returned.
The pop() method has similar behavior to shift()
, but applied to the last element in an array.
The shift()
method is a mutating method. It changes the length and the content of this
. In case you want the value of this
to be the same, but return a new array with the first element removed, you can use arr.slice(1)
instead.
The shift()
method is generic. It only expects the this
value to have a length
property and integer-keyed properties. Although strings are also array-like, this method is not suitable to be applied on them, as strings are immutable.
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:
const myFish = ["angel", "clown", "mandarin", "surgeon"];
console.log("myFish before:", myFish);
// myFish before: ['angel', 'clown', 'mandarin', 'surgeon']
const 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:
const names = ["Andrew", "Tyrone", "Paul", "Maria", "Gayatri"];
while (typeof (i = names.shift()) !== "undefined") {
console.log(i);
}
// Andrew, Tyrone, Paul, Maria, Gayatri
Calling shift() on non-array objects
The shift()
method reads the length
property of this
. If the normalized length is 0, length
is set to 0
again (whereas it may be negative or undefined
before). Otherwise, the property at 0
is returned, and the rest of the properties are shifted left by one. The length
property is decremented by one.
const arrayLike = {
length: 3,
unrelated: "foo",
2: 4,
};
console.log(Array.prototype.shift.call(arrayLike));
// undefined, because it is an empty slot
console.log(arrayLike);
// { '1': 4, length: 2, unrelated: 'foo' }
const plainObj = {};
// There's no length property, so the length is 0
Array.prototype.shift.call(plainObj);
console.log(plainObj);
// { length: 0 }