JavaScript - string methods

1. toUpperCase() and toLowerCase()

These methods change the case of a string.

let text = "Hello World";

console.log(text.toUpperCase()); // "HELLO WORLD"
console.log(text.toLowerCase()); // "hello world"

2. slice(), substring(), substr()

These methods extract parts of a string.

  • slice(start, end) → extracts from start to end (not including end). Supports negative indices.

  • substring(start, end) → similar to slice but does not support negative indices.

  • substr(start, length) → extracts length characters from start.

let str = "JavaScript";

// slice
console.log(str.slice(0, 4));      // "Java"
console.log(str.slice(-6, -3));    // "Scr"

// substring
console.log(str.substring(0, 4));  // "Java"
console.log(str.substring(3, 7));  // "aScr"

// substr
console.log(str.substr(4, 6));     // "Script"

3. replace() and replaceAll()

  • replace(searchValue, newValue) → replaces the first occurrence.

  • replaceAll(searchValue, newValue) → replaces all occurrences.

let msg = "I like cats. Cats are cute.";

// replace first occurrence
console.log(msg.replace("Cats", "Dogs")); // "I like cats. Dogs are cute."

// replace all occurrences
console.log(msg.replaceAll("Cats", "Dogs")); // "I like cats. Dogs are cute."

4. split() and join()

  • split(separator) → splits a string into an array.

  • join(separator) → joins array elements into a string.

let names = "Alice,Bob,Charlie";
let arr = names.split(","); // ["Alice", "Bob", "Charlie"]

let joined = arr.join(" & "); 
console.log(joined); // "Alice & Bob & Charlie"

5. includes(), startsWith(), endsWith()

  • includes(substring) → checks if a string contains a substring.

  • startsWith(substring) → checks if it starts with a substring.

  • endsWith(substring) → checks if it ends with a substring.

let phrase = "Hello JavaScript";

console.log(phrase.includes("Java"));      // true
console.log(phrase.startsWith("Hello"));   // true
console.log(phrase.endsWith("Script"));    // true

6. trim()

Removes whitespace from both ends of a string.

let userInput = "   hello world   ";
console.log(userInput.trim()); // "hello world"