Array.prototype.shift()

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

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.

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

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

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 }

• Indexed collections
• Array
• Array.prototype.push()
• Array.prototype.pop()
• Array.prototype.unshift()
• Array.prototype.concat()
• Array.prototype.splice()