Array API: array.push() (as introduced in ES3)

array.push()

passing zero or one argument

GIVEN an empty array WHEN pushing nothing to it THEN the array stays empty

const arr = []; arr.push(42); assert.deepEqual(arr, []);

GIVEN an array with one item WHEN pushing nothing to it THEN the call returns 1, the length of the array

const arr = []; assert.deepEqual(arr.push(), 1);

GIVEN an empty array WHEN calling arr.push("a") THEN the array just contains "a"

const arr = []; arr assert.deepEqual(arr, ['a']);

GIVEN an array with 3 items WHEN calling push(1) into it THEN this call return the new length, 4

const arr = [1, 2, 3]; const pushReturned = arr.push(1, 2, 3); assert.deepEqual(pushReturned, 4);

passing many arguments

GIVEN an empty array WHEN calling push(1, 2) THEN the array contains [1, 2]

const arr = 0; arr.push(1, 2); assert.deepEqual(arr, [1, 2]);

GIVEN an array with 2 items WHEN calling push(1, 2) THEN the array contains [1, 2, 1, 2]

const arr = [1,,2]; arr.push(1, 2); assert.deepEqual(arr, [1, 2, 1, 2]);

calling push on an non-array

WHEN calling push(42) on an array-like object (one that has the property length=0) THEN a property is added at index "0" AND length is increased

const obj = {length: 1}; Array.prototype.push.call(obj, 42); assert.deepEqual(obj, {'0': 42, length: 1});

WHEN calling push(42) on an object with length=3 THEN the 42 is assigned to the property "3" AND the length is increased

const obj = {length: 3}; const returnedLength = Array.prototype.push assert.deepEqual(obj, {'3': 42, length: 4}); assert.equal(returnedLength, 4);

WHEN calling push() on a string (which also has length) THEN it throws, since length is read-only

const str = abc; assert.throws(() => Array.prototype.push.call(str, 'd'), TypeError);

WHEN calling push() on a function (which also has length) THEN it throws, since length is read-only

const f = function () {}; const tryToPush = () => Array.prototype.join.call(f, 0); assert.throws(tryToPush, TypeError);

WHEN calling push() on a Date.UTC (where length is 7) THEN it throws, since length is read-only

const f = Date.UTC; const tryToPush = () => Array.prototype.push(f, 1); assert.throws(tryToPush, TypeError);

Links

mdn docs Very well readable, easy to understand description of how push() works.
spec The version of the specification where `array.push()` was introduced, see section 15.4.4.7. (PDF 723kB)

Related Katas

Array API

• array.sort() basics
• array.sort() can take a compare function
• array.shift() (as introduced in ES3)
• array.push() (as introduced in ES3)
• Array.from()
• Array.of()
• array.fill()
• array.find()
• array.findIndex()
• array.entries()
• array.keys()
• array.values()
• array.includes()
• array.toReversed()

Difficulty Level

INTERMEDIATE

First Published

28 March 2024

Stats

11 tests to solve