Array Methods in JS – Part 2

JavaScript Array Search Methods

What are Array Search Methods?

Array Search Methods are used to:

• Find the position (index) of an element.
• Check whether an element exists.
• Retrieve an element that satisfies a condition.
• Find the index of an element that matches a condition.
• Search from the beginning or the end of an array.

Common Array Search Methods

Method Purpose Returns indexOf() Finds the first occurrence of a value Index or -1 lastIndexOf() Finds the last occurrence of a value Index or -1 includes() Checks whether a value exists true / false find() Finds the first matching element Element or undefined findIndex() Finds the index of the first matching element Index or -1 findLast() (ES2023) Finds the last matching element Element or undefined findLastIndex() (ES2023) Finds the last matching index Index or -1

1. Array.indexOf()

Definition

The indexOf() method searches an array for a specified value and returns the index of its first occurrence.

If the value is not found, it returns -1.

Syntax

array.indexOf(searchElement)

array.indexOf(searchElement, startIndex)

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Parameters

Parameter Description searchElement Value to search for startIndex (optional) Index where the search starts

Returns

• Index of the first matching element.
• -1 if not found.

Internal Working

Suppose:

let fruits = ["Apple", "Orange", "Mango", "Orange"];

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Memory:

Index

0 → Apple
1 → Orange
2 → Mango
3 → Orange

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When:

fruits.indexOf("Orange");

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JavaScript starts from index 0:

• Apple ❌
• Orange ✅ Found

Stops immediately and returns:

1

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Example

let fruits = ["Apple", "Orange", "Banana"];

console.log(fruits.indexOf("Orange"));

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Output

1

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Example – Not Found

let fruits = ["Apple", "Orange"];

console.log(fruits.indexOf("Mango"));

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Output

-1

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Example – Start Position

let fruits = ["Apple", "Orange", "Banana", "Orange"];

console.log(fruits.indexOf("Orange", 2));

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Output

3

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Real-Time Example

Suppose an e-commerce site wants to know whether a product category exists.

let categories = ["Mobiles", "Laptops", "TV"];

let position = categories.indexOf("Laptops");

console.log(position);

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Output

1

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Time Complexity [TBD]

Case Complexity Best O(1) Worst O(n)

Common Mistake

if (arr.indexOf("Apple"))

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Wrong.

If Apple is at index 0, JavaScript treats 0 as false.

Correct:

if (arr.indexOf("Apple") !== -1)

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Interview Questions

Q. What does indexOf() return if the value is not found?

-1

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2. Array.lastIndexOf()

Definition

The lastIndexOf() method searches from the end of the array and returns the last matching index.

Syntax

array.lastIndexOf(searchElement)

array.lastIndexOf(searchElement, fromIndex)

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Parameters

Parameter Description searchElement Value to search fromIndex (optional) Index to start searching backward

Returns

Last matching index or -1.

Internal Working

let colors = ["Red", "Blue", "Green", "Blue"];

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Memory:

0 → Red
1 → Blue
2 → Green
3 → Blue

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JavaScript starts from the last index:

• Blue ✅

Returns:

3

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Example

let colors = ["Red", "Blue", "Green", "Blue"];

console.log(colors.lastIndexOf("Blue"));

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Output

3

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Real-Time Example

Suppose a browser stores recently visited pages.

let history = ["Home", "Products", "Cart", "Products"];

console.log(history.lastIndexOf("Products"));

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Output

3

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Time Complexity

O(n)

Best Practice

Use lastIndexOf() when duplicate values exist and you need the most recent occurrence.

3. Array.includes()

Definition

The includes() method checks whether a value exists in an array.

It returns:

• true
• false

Unlike indexOf(), you don’t have to compare with -1.

Syntax

array.includes(value)

array.includes(value, startIndex)

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Parameters

Parameter Description value Value to search startIndex (optional) Starting position

Returns

Boolean

Internal Working

Suppose:

let skills = ["HTML", "CSS", "JavaScript"];

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JavaScript compares each element.

If found:

true

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Else:

false

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Example

let skills = ["HTML", "CSS", "JavaScript"];

console.log(skills.includes("CSS"));

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Output

true

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Example

console.log(skills.includes("Python"));

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Output

false

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Real-Time Example

Netflix Premium Features

let features = ["4K", "HDR", "Offline Download"];

if (features.includes("HDR")) {
    console.log("HDR Supported");
}

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Output

HDR Supported

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Time Complexity

O(n)

Common Mistake

Using:

indexOf() != -1

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instead of

includes()

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includes() is more readable when you only need to know whether the value exists.

Interview Question

Which is better?

includes()

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or

indexOf()

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Use:

• includes() → existence check.
• indexOf() → position required.

4. Array.find()

Definition

The find() method returns the first element that satisfies a given condition.

If no element matches, it returns undefined.

Syntax

array.find(callback)

array.find(callback, thisArg)

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Callback Parameters

(element, index, array)

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Parameter Description element Current element index Current index array Original array

Returns

Matching element or undefined.

Internal Working

Suppose:

let numbers = [10, 25, 30, 40];

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Callback:

num > 20

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JavaScript checks:

10 ❌
25 ✅

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Stops immediately.

Returns:

25

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Example

let numbers = [10, 25, 30, 40];

let result = numbers.find(num => num > 20);

console.log(result);

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Output

25

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Real-Time Example

Find the first student who scored above 90.

let marks = [65, 72, 95, 88];

let topper = marks.find(mark => mark > 90);

console.log(topper);

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Output

95

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Time Complexity

O(n)

Common Mistake

find() returns the element, not its index.

Interview Question

Difference between find() and filter()?

• find() → first matching element.
• filter() → all matching elements.

5. Array.findIndex()

Definition

Returns the index of the first element satisfying the condition.

If not found:

-1

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Syntax

array.findIndex(callback)

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Example

let ages = [15, 18, 25, 40];

console.log(ages.findIndex(age => age >= 18));

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Output

1

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Real-Time Example

Locate the first pending order.

let orders = [
    { id: 1, status: "Completed" },
    { id: 2, status: "Pending" },
    { id: 3, status: "Pending" }
];

let index = orders.findIndex(order => order.status === "Pending");

console.log(index);

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Output

1

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Time Complexity

O(n)

6. Array.findLast() (ES2023)

Definition

Returns the last element that satisfies a condition.

Syntax

array.findLast(callback)

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Example

let numbers = [12, 18, 25, 30];

let result = numbers.findLast(num => num > 20);

console.log(result);

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Output

30

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Real-Time Example

Find the latest completed transaction.

let transactions = [
    { id: 1, status: "Pending" },
    { id: 2, status: "Completed" },
    { id: 3, status: "Completed" }
];

let latest = transactions.findLast(t => t.status === "Completed");

console.log(latest);

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Output

{ id: 3, status: "Completed" }

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Time Complexity

O(n)

7. Array.findLastIndex() (ES2023)

Definition

Returns the index of the last matching element.

Syntax

array.findLastIndex(callback)

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Example

let numbers = [5, 15, 25, 35];

console.log(numbers.findLastIndex(num => num > 20));

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Output

3

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Real-Time Example

Find the index of the last product that is out of stock.

let products = [
    { name: "Laptop", stock: 5 },
    { name: "Mouse", stock: 0 },
    { name: "Keyboard", stock: 10 },
    { name: "Monitor", stock: 0 }
];

let index = products.findLastIndex(product => product.stock === 0);

console.log(index);

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Output

3

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Time Complexity

O(n)

Summary Comparison

Method Returns Search Direction Callback Required Modifies Array indexOf() First matching index Left → Right ❌ ❌ lastIndexOf() Last matching index Right → Left ❌ ❌ includes() true / false Left → Right ❌ ❌ find() First matching element Left → Right ✅ ❌ findIndex() First matching index Left → Right ✅ ❌ findLast() Last matching element Right → Left ✅ ❌ findLastIndex() Last matching index Right → Left ✅ ❌

Tips

• Use includes() when you only need to check if a value exists.
• Use indexOf() or lastIndexOf() when you need the position of a primitive value.
• Use find() or findIndex() for arrays of objects or custom conditions.
• Use findLast() and findLastIndex() (ES2023) when you need to search from the end without reversing the array.
• Remember that indexOf() and includes() use strict equality (===) for comparison, so searching for objects compares references, not object contents.

References:
https://www.w3schools.com/js/js_array_search.asp