Strings in JavaScript for DSA

Written by Swapnil RajaPublished 28 Aug 2026

JavaScript strings cannot be changed. Every operation that looks like editing one is really building a new one, and that single fact explains most string performance problems.

Immutable means copied

You cannot assign to a character - s[0] = "x" silently does nothing. Any change produces a whole new string.

Building a string, the slow way

javascript

// A new string on every iteration.
function repeatSlow(char, times) {
  let out = ""
  for (let i = 0; i < times; i++) out += char
  return out
}

// Collect the pieces, join once at the end.
function repeatFast(char, times) {
  const parts = []
  for (let i = 0; i < times; i++) parts.push(char)
  return parts.join("")
}

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Frequency and anagrams

javascript

function counts(s) {
  const map = new Map()
  for (const ch of s) {
    map.set(ch, (map.get(ch) || 0) + 1)
  }
  return map
}

function isAnagram(a, b) {
  if (a.length !== b.length) return false

  const seen = counts(a)

  for (const ch of b) {
    if (!seen.has(ch)) return false
    seen.set(ch, seen.get(ch) - 1)
    if (seen.get(ch) === 0) seen.delete(ch)
  }

  return seen.size === 0
}

console.log(isAnagram("listen", "silent"))  // true

Where length lies

javascript

const s = "a😀b"

console.log(s.length)        // 4 — the emoji counts as 2
console.log([...s].length)   // 3 — what a person would say
console.log(s[1])            // half of the emoji, on its own

Strings in JavaScript for DSA

Immutable means copied

Costs worth memorising

Counting characters

One thing that will catch you out