Every string method returns a new string (or a value) — the original never changes.

Cheat sheet

Method	Purpose	Example → result
`includes / startsWith / endsWith`	Search → boolean	`'hello'.startsWith('he')` → `true`
`indexOf / lastIndexOf`	Position or `-1`	`'banana'.indexOf('a')` → `1`
`slice(start, end)`	Substring; negatives count from end	`'hello'.slice(-3)` → `'llo'`
`toUpperCase / toLowerCase`	Case	`'Hi'.toUpperCase()` → `'HI'`
`trim / trimStart / trimEnd`	Remove whitespace	`' x '.trim()` → `'x'`
`padStart / padEnd`	Pad to a length	`'5'.padStart(3, '0')` → `'005'`
`repeat(n)`	Repeat	`'ab'.repeat(3)` → `'ababab'`
`split(sep)`	String → array	`'a,b'.split(',')` → `['a', 'b']`
`replace / replaceAll`	Substitute (string or regex)	`'a-a'.replaceAll('-', '+')`
`at(i)`	Index with negatives	`'abc'.at(-1)` → `'c'`
`localeCompare`	Locale-aware compare for sorting	`'a'.localeCompare('b')` → `-1`

In action

JavaScript

const text = "  The quick brown fox  ";
const t = text.trim();
console.log(`[${t}]`, t.length);
console.log(t.includes("quick"), t.indexOf("o"), t.lastIndexOf("o"));
console.log(t.split(" "), t.split(" ", 2));
console.log(t.replace("o", "0"), "|", t.replaceAll("o", "0"));
console.log(t.replace(/o/g, (m) => m.toUpperCase()));
console.log("7".padStart(3, "0"), "ab".padEnd(5, "."), "=".repeat(5));

Output

[The quick brown fox] 19
true 12 17
[ 'The', 'quick', 'brown', 'fox' ] [ 'The', 'quick' ]
The quick br0wn fox | The quick br0wn f0x
The quick brOwn fOx
007 ab... =====

Common interview manipulations

JavaScript

const reverse = (s) => [...s].reverse().join("");
const isPalindrome = (s) => {
  const clean = s.toLowerCase().replace(/[^a-z0-9]/g, "");
  return clean === reverse(clean);
};
const capitalize = (s) => s.charAt(0).toUpperCase() + s.slice(1);
const titleCase = (s) => s.split(" ").map(capitalize).join(" ");
const countChars = (s) =>
  [...s].reduce((acc, ch) => ({ ...acc, [ch]: (acc[ch] ?? 0) + 1 }), {});

console.log(reverse("hello"));
console.log(isPalindrome("A man, a plan, a canal: Panama"));
console.log(titleCase("learn via notes"));
console.log(countChars("banana"));

Output

olleh
true
Learn Via Notes
{ b: 1, a: 3, n: 2 }

substring vs slice vs substr

Method	Negative indices	start > end	Status
`slice(s, e)`	Count from end	Returns `''`	Use this
`substring(s, e)`	Treated as `0`	Swaps the arguments	OK
`substr(s, len)`	Start may be negative	Second arg is a length	Deprecated

How do you reverse a string in JavaScript?