Strings

JavaScript · Reference cheat sheet

📋 Overview

Strings are immutable UTF-16 sequences. Prefer template literals, includes/startsWith, and replaceAll over older index loops. For full Unicode graphemes, be aware of code units vs code points.

🔧 Core concepts

Create / access

API	Notes
`'...'`, `"..."`, `tmpl $\{x\}`	Prefer templates for interpolation
`String(x)`, `String.raw`…``	Coerce; raw skips escape processing
`str.length`	UTF-16 code units
`str[i]`, `str.at(i)`, `str.charAt(i)`	`at` supports negatives
`charCodeAt` / `codePointAt`	Unit vs full code point

Search / match

Method	Notes
`includes(sub, from?)`	Boolean substring
`indexOf` / `lastIndexOf`	Index or `-1`
`startsWith` / `endsWith`	Prefix / suffix (pos optional)
`search(re)`	First regex match index
`match(re)`	Match array or `null`
`matchAll(re)`	Iterator of all matches (`/g` required)

Slice / split / join

Method	Notes
`slice(start?, end?)`	Supports negatives; preferred
`substring(a, b)`	Swaps if `a > b`; no negatives
`substr(start, len)`	Legacy — avoid
`split(sep\|re, limit?)`	→ array; empty sep → chars
`Array` `.join(sep)`	Inverse of `split`

Case / trim / pad / repeat

Method	Notes
`toLowerCase` / `toUpperCase`	Locale-insensitive
`toLocaleLowerCase` / `…UpperCase`	Locale-aware
`trim` / `trimStart` / `trimEnd`	Whitespace (Unicode)
`padStart(n, fill?)` / `padEnd`	Pad to length `n`
`repeat(n)`	Concatenate `n` times

Replace / normalize / unicode

Method	Notes
`replace(pat, rep\|fn)`	First match only if string/`/g` missing
`replaceAll(pat, rep\|fn)`	All; string or global regex
`normalize("NFC"\|"NFD"\|…)`	Unicode normalization forms
`localeCompare(other, locales?, opts?)`	Collation compare → −1/0/1
`for...of str`	Iterates code points
`[...str]` / `Array.from(str)`	Code-point array

const name = "Ada";
`Hello, ${name}!`;
"file.js".endsWith(".js"); // true

💡 Examples

const s = "  Hello, World  ";
s.trim().toLowerCase(); // "hello, world"
"a-b-c".split("-"); // ["a","b","c"]
"na".repeat(3); // "nanana"
"id".padStart(4, "0"); // "00id"
"foo bar foo".replaceAll("foo", "baz");

// Template multiline
const html = `
  <div class="card">
    ${title}
  </div>
`.trim();

// Code points iteration
for (const ch of "A🙂B") console.log(ch);

// matchAll
const re = /\d+/g;
for (const m of "a1 b23".matchAll(re)) {
  console.log(m[0], m.index);
}

// Slice vs substring
"abcdef".slice(1, -1); // "bcde"
"abcdef".substring(1, 4); // "bcd"

// Normalize Unicode
"é".normalize("NFC");

// Safe HTML escape (minimal)
function escapeHtml(str) {
  return str
    .replaceAll("&", "&amp;")
    .replaceAll("<", "&lt;")
    .replaceAll(">", "&gt;")
    .replaceAll('"', "&quot;");
}

⚠️ Pitfalls

Strings are immutable — methods return new strings.
replace with string pattern replaces only the first match — use replaceAll or /g.
Surrogate pairs: "🙂".length === 2; prefer for...of / codePointAt.
== coerces; compare with ===.
Building HTML via concatenation risks XSS — escape or use DOM APIs / frameworks.

encode.md — URI encoding
boolean.md — empty string falsy
arrays.md — split/join
json.md — string escaping
for_of.md — iterating characters
DOM/content_methods.md — text vs HTML

Strings

Strings

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

On this page