String Methods

The essential built-in methods for searching, transforming, and slicing strings in JavaScript.

Overview

Strings in JavaScript are immutable sequences of characters. Every string value is automatically wrapped in a `String` object, giving you access to dozens of built-in methods via the dot operator. These methods never modify the original string they always return a new value. Knowing the right string method saves you from writing tedious manual loops. Need to check if a URL contains "https"? Use `.includes()`. Need to extract a filename from a path? Use `.slice()` or `.split()`. Need to normalise user input? Chain `.trim().toLowerCase()`. Strings are also zero-indexed like arrays `"hello"[0]` is `"h"`. You can measure their length with `.length`. Individual characters can be accessed with `.charAt(i)` or bracket notation `str[i]`.

Key Points

•Strings are immutable all methods return new strings, never modifying the original
•`.length` number of characters; `str[i]` or `.charAt(i)` character at position i
•`.toUpperCase()` / `.toLowerCase()` case conversion
•`.trim()` remove leading/trailing whitespace; `.trimStart()` / `.trimEnd()` for one side
•`.includes(sub)` boolean; `.startsWith(sub)` / `.endsWith(sub)` position-specific checks
•`.indexOf(sub)` first occurrence index (−1 if not found); `.lastIndexOf(sub)` from the end
•`.slice(start, end)` extract substring from start up to (not including) end
•`.split(sep)` split string into array; `[].join(sep)` join array back into string
•`.replace(search, replacement)` first match; `.replaceAll()` all matches
•`.padStart(len, char)` / `.padEnd()` pad to target length

Examples

Case, trim, and length

const input = "  Hello, World!  ";

console.log(input.length);           // 18
console.log(input.trim());           // "Hello, World!"
console.log(input.trim().toUpperCase());  // "HELLO, WORLD!"
console.log(input.trim().toLowerCase()); // "hello, world!"

const email = "  User@Example.COM  ";
const normalised = email.trim().toLowerCase();
console.log(normalised);   // "user@example.com"

Searching inside strings

const url = "https://blog.kriativity.in/javascript/beginner/intro";

console.log(url.includes("javascript"));     // true
console.log(url.startsWith("https"));        // true
console.log(url.endsWith("intro"));          // true
console.log(url.indexOf("javascript"));      // 30  (first position)
console.log(url.indexOf("python"));          // -1  (not found)

// Check if an email looks valid
const email = "user@example.com";
console.log(email.includes("@") && email.includes(".")); // true

Slicing and splitting

const date = "2024-06-15";

// slice(start, end) end is exclusive
console.log(date.slice(0, 4));   // "2024"   (year)
console.log(date.slice(5, 7));   // "06"     (month)
console.log(date.slice(8));      // "15"     (day, no end = go to end)
console.log(date.slice(-2));     // "15"     (negative = from end)

// split and join
const parts = date.split("-");      // ["2024", "06", "15"]
console.log(parts);
console.log(parts.join("/"));        // "2024/06/15"

const sentence = "one two three";
const words = sentence.split(" ");  // ["one", "two", "three"]
console.log(words.length);          // 3

Replace, repeat, and pad

// replace first match only
let msg = "hello world, hello JS";
console.log(msg.replace("hello", "hi"));    // "hi world, hello JS"
console.log(msg.replaceAll("hello", "hi")); // "hi world, hi JS"

// Replace with regex remove all non-digits
const phone = "(555) 123-4567";
console.log(phone.replace(/\D/g, ""));      // "5551234567"

// padStart format numbers with leading zeros
const hour = "9";
console.log(hour.padStart(2, "0"));   // "09"
console.log("5".padStart(3, "0"));    // "005"

// repeat
console.log("ab".repeat(3));          // "ababab"

Common Mistakes

Modifying the return value strings are immutable; you must reassign: `str = str.trim()`, not just `str.trim()`
Using `.indexOf()` and forgetting it returns −1 (not null/undefined) when not found if (str.indexOf("x")) is truthy even when indexOf returns 0
Confusing `.slice(start, end)` with `.substring(start, end)` slice accepts negative indices, substring does not; prefer slice

Note

Chain methods for clean transformations: `input.trim().toLowerCase().replace(/\s+/g, "-")` turns " Hello World " into "hello-world" in one expression.

String Methods

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

What does `" hello ".trim()` return?