Arrays – Mutating Methods

Adds one or more elements to the end of an array, changing the original array.

Part of: JavaScript Coding without HTML and CSS — Module 24, ACARA Digital Technologies Years 9–10 — Modular and Object-Oriented Programming (JavaScript) — Module 27

Understanding Arrays – Mutating Methods in JavaScript

What it does: unshift() inserts the value at the beginning of the array. shift() removes and returns the array's first element. push() adds one or more elements to the end of the array. push, pop, shift, and unshift all mutate the original array. JavaScript uses push(), not add(), to add elements. pop() always removes the last element with no arguments needed. push() appends the given value to the end of the array. JavaScript arrays don't have an add() method - use push().

Common mistakes: Use push(), not add(), to add to the end of an array. Use pop(), not remove(), to remove the last element.

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Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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This module's key terms are also in a printable crossword (Modules 11–25) and crossword (Modules 21–30) and word search (Modules 1–29) and word search (Modules 21–52).

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Each concept is reinforced from every angle — learned, then applied, then tested.

Learning

Duo

Does push() add an element to the end of an array?

YesNo

Matches

a.add(3);

↓

Wrong - use push(), not add(), to add to the end

Applying

Meanings

Term

push()

Meaning

Adds one or more elements to the end of an array, changing the original array.

Practice

let nums = [2, 3];

Testing

Gaps

nums.(4);

Check Answer

Spots

1a.add(3);

2let a = [1, 2];

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Quick Questions

1. Does push() add an element to the end of an array?

A. Yes ✓

B. No

push() adds one or more elements to the end of the array.

2. Does pop() remove the first element of an array?

A. No ✓

B. Yes

pop() removes the last element, not the first.

3. Does unshift() add an element to the start of an array?

A. No

B. Yes ✓

unshift() adds elements at the beginning of the array.

4. Does shift() remove the first element of an array?

A. No

B. Yes ✓

shift() removes and returns the first element.

5. Do these methods change the original array?

A. Yes ✓

B. No

push, pop, shift, and unshift all mutate the original array.

Match the Pairs

nums.push(4) → Adds 4 to the end of the nums array

nums.pop() → Removes the last element from the nums array

Adds an element to the start of an array → nums.unshift(1)

Removes the first element from an array → nums.shift()

a.add(3); → Wrong - use push(), not add(), to add to the end

b.remove(); → Wrong - use pop(), not remove(), to remove the last element

Key Term

push()

Adds one or more elements to the end of an array, changing the original array.

Fill in the Blanks

1. nums.push(4);

Hint: Type the method that adds an element to the end of an array.

2. nums.pop();

Hint: Type the method that removes the last element of an array.

3. nums.unshift(1);

Hint: Type the method that adds an element to the start of an array.

4. let first = nums.shift();

Hint: Type the method that removes the first element of an array.

5. scores.push(90);

Hint: Type the method that adds an element to the end of an array.

Spot the Error

Question 1

A a.add(3);

B let a = [1, 2];

C console.log(a);

Line A — Use push(), not add(), to add to the end of an array.

Question 2

A let b = [1, 2, 3];

B b.remove();

C console.log(b);

Line B — Use pop(), not remove(), to remove the last element.

Question 3

A let c = [2, 3];

B console.log(c);

C c.push;

Line C — push() needs parentheses to actually call the method.

Question 4

A let d = [1, 2, 3];

B console.log(d);

C let e2 = 5;

D d.Pop();

Line D — 'pop' must be lowercase - a capital P causes an error.

Question 5

A let e = [2, 3];

B console.log(e);

C let f2 = 1;

D console.log(f2);

E e.Push(1);

Line E — 'push' must be lowercase - a capital P causes an error.

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