JavaScript Array Deep Dive: Push and Pop


Push and Pop are two popular methods for handling arrays in JavaScript; this short post explores both array methods.

Push

Push is used to add elements to the end of an array and returns the array’s new length. Pushing a new element into an empty array will return a value of 1 (the new length of the array).

Invoking push with no values has no effect on the original array.

let array = [1, 2];
array.push(4);
// output: 3 (new length of the array)

array
(3) [1, 2, 4]

Push can be used to add multiple values; it has variadic arity.

array.push('a', 'b', 'c');
// output: 6 (new length of the array)

array;
(6) [1,2,4,'a','b','c']

Applying Push to non-Array objects

Push is generic and can be applied to mutable Array-like objects (i.e. not strings or the arguments object).

Push relies on a length property to determine where to add new entries. If length cannot be cast to a number, push will create a new length property starting from 0.

New length property creation

var obj = {};
[].push.call(obj, 1);
console.log(obj);

// {0: 1, length: 1}

In the code above, push created the length property and incremented it.

Casting length to a number

var obj = { length: '1' }; // '1' is a string
[].push.call(obj, 1);
console.log(obj);

// {0: 1, length: 2}

Overwrite of the length property

var obj = { length: 'a' };  // 'a' is not coercible
[].push.call(obj, 1);
console.log(obj);

// {0: 1, length: 1}

Pop

Pop is the inverse of Push; it removes values from the end of an array. However unlike push, pop returns the value that was popped off and not the length of the array. Pop is a nullary function: its arity is zero.

Pop invocations on empty arrays will return undefined.

Surprising asymmetry in the JavaScript language design

let popArray = [1, 2, 4];
popArray.pop();
// output: 4 (the rightmost element in the array)

array
(2) [1, 2]

Applying Pop to non-Array objects

Pop is generic (like push) and can be similarly applied to array-like objects.

New length property creation

var obj = {};
[].pop.call(obj);
console.log(obj);

// {length: 0}

In the code above, push created the length property.

Casting length to a number

var obj = { length: '1' }; // '1' is a string
[].pop.call(obj);
// pop
console.log(obj);
// { length: 0 }

Overwrite of the length property

var obj = { length: 'a', 0:'pop' };
[].pop.call(obj);
// undefined
console.log(obj);
// {0: 'pop', length: 0}

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