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Comprehensive guide to all JavaScript array methods: push, pop, splice, slice, sort, flat, map, filter, reduce, ES2023 methods with examples, outputs, and performance tips.
JavaScript arrays come with a rich set of built-in methods for adding, removing, searching, transforming, and iterating over elements. This guide covers every array method with syntax, examples, and outputs.
Adding and Removing Elements
Example
┌────────────────────────────────────────────────────────────┐
│ ARRAY: Adding & Removing Elements │
│ │
│ unshift() ──→ [ elem1, elem2, elem3, ..., elemN ] ←── push() │
│ shift() ←── [ elem1, elem2, elem3, ..., elemN ] ──→ pop() │
│ │
│ ┌─ splice(index, deleteCount, ...items) ─┐ │
│ │ Can INSERT, DELETE, or REPLACE at any │ │
│ │ position within the array │ │
│ └─────────────────────────────────────────┘ │
└────────────────────────────────────────────────────────────┘
push() — Add to End
Example
// Syntax: arr.push(element1, element2, ..., elementN)
// Returns: new length of the array
// Mutates: YES
const fruits = ["apple", "banana"];
const newLength = fruits.push("cherry", "date");
console.log(fruits); // ["apple", "banana", "cherry", "date"]
console.log(newLength); // 4
// Push an array (adds as single element)
fruits.push(["elderberry", "fig"]);
console.log(fruits); // ["apple", "banana", "cherry", "date", ["elderberry", "fig"]]
// Push spread (adds individual elements)
const more = ["grape", "honeydew"];
fruits.push(...more);
console.log(fruits.length); // 7
Output
["apple", "banana", "cherry", "date"] 4 ["apple", "banana", "cherry", "date", ["elderberry", "fig"]] 7
pop() — Remove from End
Example
// Syntax: arr.pop()
// Returns: the removed element (or undefined if empty)
// Mutates: YES
const stack = [1, 2, 3, 4, 5];
const removed = stack.pop();
console.log(removed); // 5
console.log(stack); // [1, 2, 3, 4]
// Pop from empty array
const empty = [];
console.log(empty.pop()); // undefined
Output
5 [1, 2, 3, 4] undefined
unshift() — Add to Beginning
Example
// Syntax: arr.unshift(element1, element2, ..., elementN)
// Returns: new length of the array
// Mutates: YES
// Performance: O(n) — re-indexes all existing elements
const arr = [3, 4, 5];
const len = arr.unshift(1, 2);
console.log(arr); // [1, 2, 3, 4, 5]
console.log(len); // 5
// ⚠️ Order matters - elements are added as a group
const arr2 = [4, 5];
arr2.unshift(1, 2, 3);
console.log(arr2); // [1, 2, 3, 4, 5] — inserted in order
Output
[1, 2, 3, 4, 5] 5 [1, 2, 3, 4, 5]
shift() — Remove from Beginning
Example
// Syntax: arr.shift()
// Returns: the removed element (or undefined if empty)
// Mutates: YES
// Performance: O(n) — re-indexes all remaining elements
const queue = ["first", "second", "third"];
const first = queue.shift();
console.log(first); // "first"
console.log(queue); // ["second", "third"]
Output
"first" ["second", "third"]
splice() — The Swiss Army Knife
Example
// Syntax: arr.splice(start, deleteCount, item1, item2, ...)
// Returns: array of deleted elements
// Mutates: YES
const arr = [1, 2, 3, 4, 5];
// Delete elements
const deleted = arr.splice(1, 2); // Remove 2 elements starting at index 1
console.log(deleted); // [2, 3]
console.log(arr); // [1, 4, 5]
// Insert elements (deleteCount = 0)
arr.splice(1, 0, "a", "b");
console.log(arr); // [1, "a", "b", 4, 5]
// Replace elements
arr.splice(1, 2, "x", "y", "z");
console.log(arr); // [1, "x", "y", "z", 4, 5]
// Negative index (count from end)
const arr2 = [1, 2, 3, 4, 5];
arr2.splice(-2, 1); // Remove 1 element at index -2 (4th element)
console.log(arr2); // [1, 2, 3, 5]
// Remove all from index
const arr3 = [1, 2, 3, 4, 5];
arr3.splice(2); // Remove everything from index 2
console.log(arr3); // [1, 2]
Output
[2, 3] [1, 4, 5] [1, "a", "b", 4, 5] [1, "x", "y", "z", 4, 5] [1, 2, 3, 5] [1, 2]
Example
┌─────────────────────────────────────────────────────────────┐
│ splice() — Visual Example │
│ │
│ arr = [1, 2, 3, 4, 5] │
│ │
│ arr.splice(1, 2, "a", "b", "c") │
│ │ │ └──────────────── items to insert │
│ │ └──────────────────── delete 2 elements │
│ └─────────────────────── start at index 1 │
│ │
│ Before: [ 1, 2, 3, 4, 5 ] │
│ ╰──╯ │
│ deleted → returns [2, 3] │
│ │
│ After: [ 1, "a", "b", "c", 4, 5 ] │
│ └────────────┘ │
│ inserted items │
└─────────────────────────────────────────────────────────────┘
slice() — Extract a Portion (Non-Mutating)
Example
// Syntax: arr.slice(start, end) — end is exclusive
// Returns: new array with extracted elements
// Mutates: NO
const arr = [1, 2, 3, 4, 5];
console.log(arr.slice(1, 3)); // [2, 3]
console.log(arr.slice(2)); // [3, 4, 5] — from index 2 to end
console.log(arr.slice(-2)); // [4, 5] — last 2 elements
console.log(arr.slice(-3, -1)); // [3, 4]
console.log(arr.slice()); // [1, 2, 3, 4, 5] — shallow copy
console.log(arr); // [1, 2, 3, 4, 5] — unchanged!
// Shallow copy caveat
const nested = [[1, 2], [3, 4]];
const copy = nested.slice();
copy[0].push(99);
console.log(nested[0]); // [1, 2, 99] — nested reference shared!
Output
[2, 3] [3, 4, 5] [4, 5] [3, 4] [1, 2, 3, 4, 5] [1, 2, 3, 4, 5] [1, 2, 99]
Example
┌────────────────────────────────────────────────────────────────┐
│ slice() vs splice() — Key Differences │
├────────────────────────────┬───────────────────────────────────┤
│ slice(start, end) │ splice(start, delCount, items) │
├────────────────────────────┼───────────────────────────────────┤
│ Does NOT mutate original │ MUTATES original array │
│ end is EXCLUSIVE │ deleteCount = how many to remove │
│ Returns extracted portion │ Returns DELETED elements │
│ Only 2 params (start/end) │ Can accept unlimited new items │
│ Used for copying/reading │ Used for insert/delete/replace │
└────────────────────────────┴───────────────────────────────────┘
concat() — Merge Arrays (Non-Mutating)
Example
// Syntax: arr.concat(value1, value2, ..., valueN)
// Returns: new merged array
// Mutates: NO
const a = [1, 2];
const b = [3, 4];
const c = [5, 6];
const merged = a.concat(b, c);
console.log(merged); // [1, 2, 3, 4, 5, 6]
console.log(a); // [1, 2] — unchanged
// Concat with values
const result = a.concat(3, 4, [5, 6]);
console.log(result); // [1, 2, 3, 4, 5, 6]
// Only flattens one level
const deep = a.concat([[3, 4]]);
console.log(deep); // [1, 2, [3, 4]]
Output
[1, 2, 3, 4, 5, 6] [1, 2] [1, 2, 3, 4, 5, 6] [1, 2, [3, 4]]
sort() — Sort In-Place
Example
// Syntax: arr.sort(compareFn)
// Returns: the sorted array (same reference)
// Mutates: YES
// ⚠️ Default sort converts to strings!
const nums = [10, 9, 2, 21, 100];
nums.sort();
console.log(nums); // [10, 100, 2, 21, 9] — lexicographic!
// Numeric sort
const numbers = [10, 9, 2, 21, 100];
numbers.sort((a, b) => a - b); // ascending
console.log(numbers); // [2, 9, 10, 21, 100]
numbers.sort((a, b) => b - a); // descending
console.log(numbers); // [100, 21, 10, 9, 2]
// Sort strings (case-insensitive)
const words = ["Banana", "apple", "Cherry"];
words.sort((a, b) => a.toLowerCase().localeCompare(b.toLowerCase()));
console.log(words); // ["apple", "Banana", "Cherry"]
// Sort objects by property
const people = [
{ name: "Charlie", age: 30 },
{ name: "Alice", age: 25 },
{ name: "Bob", age: 35 }
];
people.sort((a, b) => a.age - b.age);
console.log(people.map(p => p.name)); // ["Alice", "Charlie", "Bob"]
Output
[10, 100, 2, 21, 9] [2, 9, 10, 21, 100] [100, 21, 10, 9, 2] ["apple", "Banana", "Cherry"] ["Alice", "Charlie", "Bob"]
reverse() — Reverse In-Place
Example
// Syntax: arr.reverse()
// Returns: the reversed array (same reference)
// Mutates: YES
const arr = [1, 2, 3, 4, 5];
const reversed = arr.reverse();
console.log(arr); // [5, 4, 3, 2, 1]
console.log(reversed); // [5, 4, 3, 2, 1]
console.log(arr === reversed); // true (same reference!)
// Reverse without mutating
const original = [1, 2, 3];
const reversedCopy = [...original].reverse();
console.log(original); // [1, 2, 3] — unchanged
console.log(reversedCopy); // [3, 2, 1]
Output
[5, 4, 3, 2, 1] [5, 4, 3, 2, 1] true [1, 2, 3] [3, 2, 1]
flat() and flatMap()
flat() — Flatten Nested Arrays
Example
// Syntax: arr.flat(depth)
// Returns: new flattened array
// Mutates: NO
const nested = [1, [2, 3], [4, [5, 6]]];
console.log(nested.flat()); // [1, 2, 3, 4, [5, 6]] — depth 1 (default)
console.log(nested.flat(2)); // [1, 2, 3, 4, 5, 6] — depth 2
console.log(nested.flat(Infinity)); // [1, 2, 3, 4, 5, 6] — all levels
// Removes holes
const sparse = [1, , 3, , 5];
console.log(sparse.flat()); // [1, 3, 5]
Output
[1, 2, 3, 4, [5, 6]] [1, 2, 3, 4, 5, 6] [1, 2, 3, 4, 5, 6] [1, 3, 5]
flatMap() — Map + Flatten (depth 1)
Example
// Syntax: arr.flatMap(callback)
// Equivalent to: arr.map(fn).flat(1) but more efficient
// Mutates: NO
const sentences = ["Hello World", "Foo Bar"];
const words = sentences.flatMap(s => s.split(" "));
console.log(words); // ["Hello", "World", "Foo", "Bar"]
// Remove and transform simultaneously
const nums = [1, 2, 3, 4, 5, 6];
const result = nums.flatMap(n => n % 2 === 0 ? [n, n * 2] : []);
console.log(result); // [2, 4, 4, 8, 6, 12]
Output
["Hello", "World", "Foo", "Bar"] [2, 4, 4, 8, 6, 12]
fill() — Fill with Static Value
Example
// Syntax: arr.fill(value, start, end)
// Returns: the modified array
// Mutates: YES
const arr = [1, 2, 3, 4, 5];
arr.fill(0);
console.log(arr); // [0, 0, 0, 0, 0]
arr.fill(7, 1, 3);
console.log(arr); // [0, 7, 7, 0, 0]
// Initialize array with value
const zeros = new Array(5).fill(0);
console.log(zeros); // [0, 0, 0, 0, 0]
// ⚠️ Gotcha with objects — all share same reference!
const grid = new Array(3).fill([]);
grid[0].push(1);
console.log(grid); // [[1], [1], [1]] — all same array!
// ✅ Correct way
const grid2 = Array.from({ length: 3 }, () => []);
grid2[0].push(1);
console.log(grid2); // [[1], [], []]
Output
[0, 0, 0, 0, 0] [0, 7, 7, 0, 0] [0, 0, 0, 0, 0] [[1], [1], [1]] [[1], [], []]
copyWithin() — Copy Within Array
Example
// Syntax: arr.copyWithin(target, start, end)
// Returns: the modified array
// Mutates: YES
const arr = [1, 2, 3, 4, 5];
// Copy elements 3-4 to position 0
arr.copyWithin(0, 3, 5);
console.log(arr); // [4, 5, 3, 4, 5]
const arr2 = [1, 2, 3, 4, 5];
arr2.copyWithin(1, 3);
console.log(arr2); // [1, 4, 5, 4, 5]
Output
[4, 5, 3, 4, 5] [1, 4, 5, 4, 5]
Searching Methods
Example
┌─────────────────────────────────────────────────────────────────┐
│ SEARCHING METHODS — Decision Tree │
│ │
│ Need to know if value EXISTS? │
│ └─→ includes(value) → boolean │
│ │
│ Need the INDEX of a value? │
│ └─→ indexOf(value) / lastIndexOf(value) → number (-1 if none) │
│ │
│ Need the ELEMENT matching a condition? │
│ └─→ find(fn) → element | undefined │
│ │
│ Need the INDEX matching a condition? │
│ └─→ findIndex(fn) → number (-1 if none) │
│ │
│ Need ALL elements matching a condition? │
│ └─→ filter(fn) → new array │
└─────────────────────────────────────────────────────────────────┘
indexOf() and lastIndexOf()
Example
const arr = [1, 2, 3, 2, 1];
console.log(arr.indexOf(2)); // 1
console.log(arr.indexOf(2, 2)); // 3 (search from index 2)
console.log(arr.indexOf(99)); // -1
console.log(arr.lastIndexOf(2)); // 3
console.log(arr.lastIndexOf(2, 2)); // 1
// ⚠️ Uses strict equality (===)
console.log([NaN].indexOf(NaN)); // -1 (NaN !== NaN)
Output
1 3 -1 3 1 -1
includes() (ES2016)
Example
const arr = [1, 2, 3, NaN];
console.log(arr.includes(2)); // true
console.log(arr.includes(99)); // false
console.log(arr.includes(NaN)); // true (handles NaN correctly!)
// Compare with indexOf
console.log([NaN].indexOf(NaN) !== -1); // false ❌
console.log([NaN].includes(NaN)); // true ✅
Output
true false true false true
find() and findIndex()
Example
const users = [
{ id: 1, name: "Alice", age: 25 },
{ id: 2, name: "Bob", age: 30 },
{ id: 3, name: "Charlie", age: 35 }
];
// find() returns first matching element
const user = users.find(u => u.age > 28);
console.log(user); // { id: 2, name: "Bob", age: 30 }
// findIndex() returns first matching index
const index = users.findIndex(u => u.name === "Charlie");
console.log(index); // 2
// findLast() and findLastIndex() (ES2023)
const nums = [1, 2, 3, 4, 5];
console.log(nums.findLast(n => n % 2 === 0)); // 4
console.log(nums.findLastIndex(n => n % 2 === 0)); // 3
Output
{ id: 2, name: "Bob", age: 30 }
2
4
3Testing Methods
every() and some()
Example
const numbers = [2, 4, 6, 8, 10];
// every() — ALL must pass
console.log(numbers.every(n => n % 2 === 0)); // true
console.log(numbers.every(n => n > 5)); // false
// some() — at least ONE must pass
console.log(numbers.some(n => n > 8)); // true
console.log(numbers.some(n => n > 10)); // false
Output
true false true false
Conversion Methods
join()
Example
const arr = ["Hello", "World", "JavaScript"];
console.log(arr.join()); // "Hello,World,JavaScript"
console.log(arr.join(" ")); // "Hello World JavaScript"
console.log(arr.join(" - ")); // "Hello - World - JavaScript"
console.log(arr.join("")); // "HelloWorldJavaScript"
// With numbers
console.log([2024, 6, 15].join("/")); // "2024/6/15"
// Handles null/undefined as empty strings
console.log([1, null, undefined, 4].join("-")); // "1---4"
Output
Hello,World,JavaScript Hello World JavaScript Hello - World - JavaScript HelloWorldJavaScript 2024/6/15 1---4
ES2023 Immutable Methods
Example
┌───────────────────────────────────────────────────────────────────┐
│ ES2023 Immutable Counterparts │
│ │
│ MUTATING (old) → NON-MUTATING (ES2023) │
│ ───────────────── ───────────────────── │
│ arr.sort(fn) → arr.toSorted(fn) │
│ arr.reverse() → arr.toReversed() │
│ arr.splice(i, d, ...x) → arr.toSpliced(i, d, ...x) │
│ arr[index] = value → arr.with(index, value) │
│ │
│ 💡 All return NEW arrays — original is NEVER modified │
│ 💡 Perfect for React state updates & functional programming │
└───────────────────────────────────────────────────────────────────┘
toSorted() — Non-Mutating Sort
Example
const arr = [3, 1, 4, 1, 5];
const sorted = arr.toSorted((a, b) => a - b);
console.log(sorted); // [1, 1, 3, 4, 5]
console.log(arr); // [3, 1, 4, 1, 5] — unchanged!
Output
[1, 1, 3, 4, 5] [3, 1, 4, 1, 5]
toReversed() — Non-Mutating Reverse
Example
const arr = [1, 2, 3, 4, 5];
const reversed = arr.toReversed();
console.log(reversed); // [5, 4, 3, 2, 1]
console.log(arr); // [1, 2, 3, 4, 5] — unchanged!
Output
[5, 4, 3, 2, 1] [1, 2, 3, 4, 5]
toSpliced() — Non-Mutating Splice
Example
const arr = [1, 2, 3, 4, 5];
const result = arr.toSpliced(1, 2, "a", "b");
console.log(result); // [1, "a", "b", 4, 5]
console.log(arr); // [1, 2, 3, 4, 5] — unchanged!
Output
[1, "a", "b", 4, 5] [1, 2, 3, 4, 5]
with() — Non-Mutating Index Assignment
Example
const arr = [1, 2, 3, 4, 5];
const updated = arr.with(2, 99);
console.log(updated); // [1, 2, 99, 4, 5]
console.log(arr); // [1, 2, 3, 4, 5] — unchanged!
// Supports negative indices
const updated2 = arr.with(-1, 0);
console.log(updated2); // [1, 2, 3, 4, 0]
Output
[1, 2, 99, 4, 5] [1, 2, 3, 4, 5] [1, 2, 3, 4, 0]
Method Chaining
Example
┌──────────────────────────────────────────────────────────────┐
│ METHOD CHAINING — Data Pipeline │
│ │
│ products │
│ │ │
│ ▼ │
│ .filter(p => p.inStock) → remove out-of-stock │
│ │ │
│ ▼ │
│ .sort((a,b) => a.price - b.price) → cheapest first │
│ │ │
│ ▼ │
│ .map(p => `${p.name}: $${p.price}`) → format strings │
│ │ │
│ ▼ │
│ .slice(0, 3) → take top 3 │
│ │ │
│ ▼ │
│ ["Headphones: $149", "Watch: $299", "Tablet: $499"] │
└──────────────────────────────────────────────────────────────┘
Example
const products = [
{ name: "Laptop", price: 999, inStock: true },
{ name: "Phone", price: 699, inStock: false },
{ name: "Tablet", price: 499, inStock: true },
{ name: "Watch", price: 299, inStock: true },
{ name: "Headphones", price: 149, inStock: true }
];
// Chain: filter → sort → map → slice
const topAffordable = products
.filter(p => p.inStock)
.sort((a, b) => a.price - b.price)
.map(p => `${p.name}: $${p.price}`)
.slice(0, 3);
console.log(topAffordable);
// ["Headphones: $149", "Watch: $299", "Tablet: $499"]
// Chain with reduce
const totalInStock = products
.filter(p => p.inStock)
.reduce((sum, p) => sum + p.price, 0);
console.log(`Total: $${totalInStock}`);
Output
["Headphones: $149", "Watch: $299", "Tablet: $499"] Total: $1946
Performance Comparison
| Method | Time Complexity | Creates New Array | Use When |
|---|---|---|---|
| push/pop | O(1) | No | Stack operations |
| shift/unshift | O(n) | No | Queue (prefer deque for perf) |
| splice | O(n) | No (returns deleted) | Insert/delete at position |
| slice | O(k) | Yes | Extract portion |
| concat | O(n+m) | Yes | Merge arrays |
| sort | O(n log n) | No | Ordering |
| indexOf | O(n) | No | Find index of value |
| includes | O(n) | No | Check existence |
| find | O(n) | No | Find first match |
| flat | O(n) | Yes | Flatten nested |
🚫 Common Mistakes
Mistake 1: Sorting Numbers Without a Compare Function
Example
// ❌ WRONG — sorts lexicographically (as strings)
const nums = [10, 9, 2, 21, 100];
nums.sort();
console.log(nums); // [10, 100, 2, 21, 9] — NOT numeric order!
Output
[10, 100, 2, 21, 9]
Example
// ✅ CORRECT — use a compare function for numeric sort
const nums = [10, 9, 2, 21, 100];
nums.sort((a, b) => a - b);
console.log(nums); // [2, 9, 10, 21, 100]
Output
[2, 9, 10, 21, 100]
Mistake 2: Using reverse() Without Cloning
Example
// ❌ WRONG — mutates the original array
const original = [1, 2, 3, 4, 5];
const reversed = original.reverse();
console.log(original); // [5, 4, 3, 2, 1] — original is destroyed!
console.log(original === reversed); // true — same reference!
Output
[5, 4, 3, 2, 1] true
Example
// ✅ CORRECT — spread first, or use toReversed()
const original = [1, 2, 3, 4, 5];
const reversed = [...original].reverse();
// OR: const reversed = original.toReversed(); (ES2023)
console.log(original); // [1, 2, 3, 4, 5] — safe!
console.log(reversed); // [5, 4, 3, 2, 1]
Output
[1, 2, 3, 4, 5] [5, 4, 3, 2, 1]
Mistake 3: Confusing splice() and slice()
Example
// ❌ WRONG — wanted to copy but used splice (which mutates)
const arr = [1, 2, 3, 4, 5];
const portion = arr.splice(1, 3); // Deletes elements!
console.log(arr); // [1, 5] — original is modified!
console.log(portion); // [2, 3, 4]
Output
[1, 5] [2, 3, 4]
Example
// ✅ CORRECT — use slice() to extract without mutating
const arr = [1, 2, 3, 4, 5];
const portion = arr.slice(1, 4); // end is exclusive
console.log(arr); // [1, 2, 3, 4, 5] — unchanged!
console.log(portion); // [2, 3, 4]
Output
[1, 2, 3, 4, 5] [2, 3, 4]
Mistake 4: fill() with Object References
Example
// ❌ WRONG — all elements share the SAME object reference
const grid = new Array(3).fill([]);
grid[0].push("hello");
console.log(grid); // [["hello"], ["hello"], ["hello"]] — all affected!
Output
[["hello"], ["hello"], ["hello"]]
Example
// ✅ CORRECT — create unique objects with Array.from()
const grid = Array.from({ length: 3 }, () => []);
grid[0].push("hello");
console.log(grid); // [["hello"], [], []] — only first changed
Output
[["hello"], [], []]
Mistake 5: Using indexOf() to Find NaN
Example
// ❌ WRONG — indexOf uses strict equality, NaN !== NaN
const arr = [1, NaN, 3];
console.log(arr.indexOf(NaN)); // -1 (not found!)
console.log(arr.indexOf(NaN) !== -1); // false — thinks NaN isn't there
Output
-1 false
Example
// ✅ CORRECT — use includes() which handles NaN properly
const arr = [1, NaN, 3];
console.log(arr.includes(NaN)); // true ✅
// Or use findIndex with Number.isNaN
console.log(arr.findIndex(Number.isNaN)); // 1
Output
true 1
Mistake 6: Forgetting that sort() Mutates
Example
// ❌ WRONG — accidentally mutates state (e.g., in React)
const state = [5, 3, 8, 1];
const sorted = state.sort((a, b) => a - b);
console.log(state === sorted); // true — same array!
console.log(state); // [1, 3, 5, 8] — state is mutated!
Output
true [1, 3, 5, 8]
Example
// ✅ CORRECT — use toSorted() or spread + sort
const state = [5, 3, 8, 1];
const sorted = state.toSorted((a, b) => a - b); // ES2023
// OR: const sorted = [...state].sort((a, b) => a - b);
console.log(state); // [5, 3, 8, 1] — unchanged!
console.log(sorted); // [1, 3, 5, 8]
Output
[5, 3, 8, 1] [1, 3, 5, 8]
Mistake 7: Using map() When You Don't Need the Return Value
Example
// ❌ WRONG — map creates a new array that's wasted
const users = ["Alice", "Bob", "Charlie"];
users.map(user => console.log(user)); // Creates unused array of undefineds
Output
Alice Bob Charlie
Example
// ✅ CORRECT — use forEach for side effects
const users = ["Alice", "Bob", "Charlie"];
users.forEach(user => console.log(user)); // No wasted array
Output
Alice Bob Charlie
🎯 Key Takeaways
- Mutating vs Non-Mutating — Know which methods mutate (sort, reverse, splice, push, pop, shift, unshift, fill, copyWithin) vs which return new arrays (slice, map, filter, reduce, concat, flat, flatMap)
- ES2023 Immutable Alternatives —
toSorted(),toReversed(),toSpliced(),with()provide safe, non-mutating versions — prefer these in React/Redux/state management - sort() is Lexicographic by Default — Always pass
(a, b) => a - bfor numbers; default sort converts to strings first - includes() vs indexOf() for NaN —
includes()handlesNaNcorrectly;indexOf()cannot findNaNbecauseNaN !== NaN - push/pop are O(1), shift/unshift are O(n) — In performance-critical code, prefer operations at the end of arrays
- Method Chaining —
filter → map → sort → slicecreates clean, readable data pipelines; each step returns a new array - splice() vs slice() — splice = "surgery" (mutates, can add/remove/replace); slice = "photocopy" (never mutates, just extracts)
- fill() Object Trap —
new Array(n).fill({})shares ONE reference; useArray.from({length: n}, () => ({}))for independent objects - flatMap() > map().flat() —
flatMap()is more efficient (single pass) and is perfect for mapping that also filters or expands - find() vs filter() —
find()stops at first match (early exit);filter()always scans everything — usefind()when you only need one result
❓ Interview Questions
Q1. What's the difference between splice() and slice()?
splice() mutates the original array — it can add, remove, or replace elements and returns the deleted elements. slice() never mutates — it returns a new array containing a portion of the original. Remember: splice = change, slice = copy.
Q2. How does sort() work without a compare function?
It converts elements to strings and sorts lexicographically (Unicode code point order). [10, 9, 2, 21] becomes [10, 2, 21, 9] because "10" < "2" (compares character by character: "1" < "2"). Always provide a compare function for numbers.
Q3. What is the difference between find() and filter()?
find() returns the first matching element (or undefined) and stops early — O(1) best case. filter() returns an array of all matches and always scans the entire array — O(n). Use find() when you need just one result.
Q4. How do you shuffle an array randomly?
Example
// Fisher-Yates shuffle — O(n), unbiased
function shuffle(arr) {
const result = [...arr];
for (let i = result.length - 1; i > 0; i--) {
const j = Math.floor(Math.random() * (i + 1));
[result[i], result[j]] = [result[j], result[i]];
}
return result;
}
console.log(shuffle([1, 2, 3, 4, 5])); // e.g., [3, 1, 5, 2, 4]
Output
[3, 1, 5, 2, 4]
Q5. What is the difference between includes() and indexOf()?
includes() returns a boolean and correctly handles NaN (uses SameValueZero algorithm). indexOf() returns an index (or -1) and cannot find NaN because it uses strict equality (===), and NaN !== NaN.
Q6. What are ES2023 immutable array methods?
toSorted(), toReversed(), toSpliced(), and with() — non-mutating versions of sort(), reverse(), splice(), and index assignment. They all return new arrays, making them ideal for functional programming and state management (React, Redux).
Q7. When should you prefer flatMap() over map().flat()?
flatMap() is more efficient because it combines mapping and flattening in a single pass. Use it when your map function returns arrays that need to be flattened by one level. It's also great for simultaneously filtering and transforming (return [] to exclude, [value] to include).
📝 FAQ
Q: Can I use forEach() with break or return to stop early?
No. forEach() always iterates over every element — return inside the callback only skips that single iteration (like continue in a for loop). To break early, use a regular for loop, for...of with break, or methods like find()/some()/every() that have built-in short-circuiting.
Example
// ❌ Cannot break out of forEach
[1, 2, 3, 4, 5].forEach(n => {
if (n === 3) return; // Only skips this iteration
console.log(n);
});
// ✅ Use for...of to break early
for (const n of [1, 2, 3, 4, 5]) {
if (n === 3) break;
console.log(n);
}
Output
1 2 4 5 1 2
Q: What's the difference between Array.from() and the spread operator [...] for creating arrays?
Both create new arrays, but Array.from() is more powerful: it accepts array-like objects (with .length property), iterables, and a mapping function as the second argument. Spread ([...]) only works with iterables. Array.from({length: 5}, (_, i) => i) creates [0, 1, 2, 3, 4] — spread can't do this.
Example
// Array.from with array-like object (has .length)
console.log(Array.from({ length: 3 }, (_, i) => i * 2)); // [0, 2, 4]
// Spread only works with iterables
console.log([..."hello"]); // ["h", "e", "l", "l", "o"]
// [...{length: 3}] → TypeError: not iterable
Output
[0, 2, 4] ["h", "e", "l", "l", "o"]
Q: Does sort() guarantee stable sorting in JavaScript?
Yes! As of ECMAScript 2019 (ES10), Array.prototype.sort() is required to be stable — elements that compare equal retain their original relative order. All modern browsers (Chrome 70+, Firefox 3+, Safari 10.1+, Node.js 12+) implement stable sort.
Q: How do I remove duplicates from an array?
The easiest way is using Set:
Example
const arr = [1, 2, 2, 3, 3, 3, 4];
const unique = [...new Set(arr)];
console.log(unique); // [1, 2, 3, 4]
// For objects, use a Map or filter with findIndex
const users = [
{ id: 1, name: "Alice" },
{ id: 2, name: "Bob" },
{ id: 1, name: "Alice" }
];
const uniqueUsers = users.filter(
(user, index, self) => self.findIndex(u => u.id === user.id) === index
);
console.log(uniqueUsers.length); // 2
Output
[1, 2, 3, 4] 2
Q: When should I use reduce() vs a for loop?
Use reduce() for transforming an array into a single value (sum, product, grouped object, flattened array). Use a for loop when: (1) you need early termination, (2) the logic is complex with multiple accumulators, or (3) readability suffers. A common guideline: if your reduce() callback is more than 3 lines, consider a for loop instead.
Example
// ✅ Good use of reduce — simple accumulation
const sum = [1, 2, 3, 4, 5].reduce((acc, n) => acc + n, 0);
console.log(sum); // 15
// ✅ Good use of reduce — grouping
const words = ["hello", "hi", "hey", "world", "wow"];
const grouped = words.reduce((acc, word) => {
const key = word[0];
(acc[key] = acc[key] || []).push(word);
return acc;
}, {});
console.log(grouped); // { h: ["hello", "hi", "hey"], w: ["world", "wow"] }
Output
15
{ h: ["hello", "hi", "hey"], w: ["world", "wow"] }Cheat Sheet
Example
┌──────────────────────────────────────────────────────────────────┐
│ JAVASCRIPT ARRAY METHODS — QUICK REFERENCE │
├──────────────────────────────────────────────────────────────────┤
│ │
│ Add end: push(item) → new length │
│ Remove end: pop() → removed item │
│ Add front: unshift(item) → new length │
│ Remove front: shift() → removed item │
│ Edit anywhere: splice(idx, del, ...items) │
│ Copy portion: slice(start, end) → new array │
│ Merge: concat(arr2) → new array │
│ Sort (num): sort((a,b) => a-b) → same array │
│ Reverse: [...arr].reverse() → no mutation │
│ Flatten: flat(depth) → new array │
│ Find one: find(fn) → element │
│ Find all: filter(fn) → new array │
│ Check exists: includes(val) → boolean │
│ Index of: indexOf(val) → number │
│ Every match: every(fn) → boolean │
│ Any match: some(fn) → boolean │
│ Sum all: reduce((acc,v)=>acc+v, 0) │
│ Join to str: join(", ") → string │
│ │
│ ─── ES2023 (Non-Mutating) ─── │
│ toSorted(fn) → like sort(), returns new │
│ toReversed() → like reverse(), returns new │
│ toSpliced(i,d,x) → like splice(), returns new │
│ with(idx, val) → like arr[idx]=val, returns new │
│ │
└──────────────────────────────────────────────────────────────────┘
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