JavaScript Array Methods Cheat Sheet

➕ push()

Adds item(s) to the end Returns new length of array Modifies the original array

const nums = [1, 2];
nums.push(3); // [1, 2, 3]

➖ pop()

• Removes last item
• Returns removed value
• Modifies the original array

const nums = [1, 2, 3];
const last = nums.pop(); // 3

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⬅️ shift()

• Removes first item
• Returns removed value
• Modifies the original array

const nums = [1, 2, 3];
const first = nums.shift(); // 1

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➡️ unshift()

• Adds item(s) to start
• Returns new length
• Modifies the original array

const nums = [2, 3];
nums.unshift(1); // [1, 2, 3]

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✂️ splice()

• Add/remove at any index
• Modifies original array
• Returns removed items

const nums = [1, 2, 3, 4];
nums.splice(1, 2); // [2, 3]

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🧬 slice()

• Returns a shallow copy
• Doesn’t modify original
• Use for partial clones

const nums = [1, 2, 3, 4];
const copy = nums.slice(1, 3); // [2, 3]

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🔗 concat()

• Combines arrays
• Returns a new array
• Original arrays stay unchanged

const a = [1, 2];
const b = [3, 4];
const result = a.concat(b); // [1, 2, 3, 4]

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🧵 join()

• Converts array to string
• Takes a separator (like -, ,, or '')
• Doesn’t mutate array

const items = ['a', 'b', 'c'];
const str = items.join('-'); // 'a-b-c'

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🗺️ map()

• Transforms each item
• Returns a new array
• Original array stays unchanged

const nums = [1, 2, 3];
const doubled = nums.map(n => n * 2); // [2, 4, 6]

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🔍 filter()

• Keeps items matching condition
• Returns new filtered array
• Original array stays unchanged

const nums = [1, 2, 3, 4];
const evens = nums.filter(n => n % 2 === 0); // [2, 4]

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➕ reduce()

• Reduces array to one value
• Good for sums, counts, etc.
• Original array stays unchanged

const nums = [1, 2, 3, 4];
const sum = nums.reduce((acc, n) => acc + n, 0); // 10

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🔎 find()

• Returns first match or undefined
• Doesn’t modify array
• Good for searching one item

const nums = [1, 2, 3];
const found = nums.find(n => n > 1); // 2

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🔢 findIndex()

• Returns index of first match
• Returns -1 if not found
• Doesn’t modify array

const nums = [1, 2, 3];
const idx = nums.findIndex(n => n === 2); // 1

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✔️ some()

• Checks if any item matches
• Returns boolean
• Doesn’t modify array

const nums = [1, 2, 3];
const hasEven = nums.some(n => n % 2 === 0); // true

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✅ every()

• Checks if all items match
• Returns boolean
• Doesn’t modify array

const nums = [2, 4, 6];
const allEven = nums.every(n => n % 2 === 0); // true

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🔢 indexOf()

• Returns first index of item
• Returns -1 if not found
• Doesn’t modify array

const fruits = ['apple', 'banana', 'orange'];
const idx = fruits.indexOf('banana'); // 1

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🕵️‍♂️ includes()

• Checks if array contains item
• Returns true or false
• Doesn’t modify array

const fruits = ['apple', 'banana'];
const hasApple = fruits.includes('apple'); // true

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🔄 reverse()

• Reverses the order of elements
• Modifies the original array
• Use .slice().reverse() to copy

const nums = [1, 2, 3];
nums.reverse(); // [3, 2, 1]
const copy = nums.slice().reverse(); // reversed copy

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🔢 sort()

• Sorts array in place
• Modifies original array
• Use compare fn for custom sorting

const nums = [3, 1, 2];
nums.sort(); // [1, 2, 3]
nums.sort((a, b) => b - a); // [3, 2, 1]

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🧩 forEach()

• Runs a function on each item
• No returned array
• Good for side effects, logging

const nums = [1, 2, 3];
nums.forEach(n => console.log(n));

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📜 Array.from()

• Creates array from iterable/array-like
• Can map values during creation
• Doesn’t modify input

const str = 'hello';
const arr = Array.from(str); // ['h','e','l','l','o']

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✨ Spread `...`

• Spreads elements into new array
• Useful for cloning and merging
• Doesn’t mutate originals

* const a = [1, 2];
* const b = [3, 4];
* const combined = [...a, ...b]; // [1, 2, 3, 4]

📄 Quick Reference

Method	Returns	Example	Mutates Original Array?
push()	New length	[1, 2].push(3) → 3	✅
pop()	Removed element	[1, 2, 3].pop() → 3	✅
shift()	Removed element	[1, 2, 3].shift() → 1	✅
unshift()	New length	[2, 3].unshift(1) → 3	✅
splice()	Removed elements	[1, 2, 3, 4].splice(1, 2) → [2, 3]	✅
slice()	New array	[1, 2, 3, 4].slice(1, 3) → [2, 3]	❌
concat()	New array	[1, 2].concat([3]) → [1, 2, 3]	❌
join()	String	['a', 'b'].join('-') → 'a-b'	❌
map()	New array	[1, 2].map(x => x * 2) → [2, 4]	❌
filter()	New array	[1, 2].filter(x => x > 1) → [2]	❌
reduce()	Single value	[1, 2].reduce((a, b) => a + b) → 3	❌
sort()	Sorted array	[3, 1, 2].sort() → [1, 2, 3]	✅
reverse()	Reversed array	[1, 2, 3].reverse() → [3, 2, 1]	✅
includes()	Boolean	[1, 2, 3].includes(2) → true	❌
find()	First matching item	[1, 2].find(x => x > 1) → 2	❌
findIndex()	Index of first match	[1, 2, 3].findIndex(x => x === 2) → 1	❌
some()	Boolean	[1, 2, 3].some(x => x > 2) → true	❌
every()	Boolean	[1, 2, 3].every(x => x > 0) → true	❌
flat()	New flattened array	[1, [2, 3]].flat() → [1, 2, 3]	❌
flatMap()	New mapped & flat	[1, 2].flatMap(x => [x, x*2]) → [1, 2, 2, 4]	❌
fill()	Modified array	[1, 2, 3].fill(0, 1) → [1, 0, 0]	✅
copyWithin()	Modified array	[1, 2, 3, 4].copyWithin(1, 2) → [1, 3, 4, 4]	✅