switch / case

JavaScript · Reference cheat sheet

📋 Overview

switch selects a branch by strict equality (===) against a discriminant. Cases fall through until break, return, or throw. Prefer maps/objects or early returns when branches share little structure.

🔧 Core concepts

Syntax: switch (expr) \{ case v: ... break; default: ... \}.
Matching: strict equality; no pattern matching built-in.
Fall-through: intentional sharing of bodies; always comment intentional fall-through.
Scope: wrap case bodies in \{ \} when using let/const to avoid TDZ clashes.
Alternatives: lookup tables, Map, if/else chains, pattern libs.

switch (status) {
  case 200:
    return "ok";
  case 404:
    return "missing";
  default:
    return "other";
}

💡 Examples

function httpLabel(code) {
  switch (code) {
    case 200:
    case 201:
    case 204:
      return "success";
    case 301:
    case 302:
      return "redirect";
    case 400:
    case 401:
    case 403:
    case 404:
      return "client-error";
    case 500:
      return "server-error";
    default:
      return "unknown";
  }
}

// Block scope per case
switch (kind) {
  case "user": {
    const id = crypto.randomUUID();
    return { kind, id };
  }
  case "guest": {
    const id = "guest";
    return { kind, id };
  }
  default:
    throw new Error(`bad kind: ${kind}`);
}

// Dispatch table
const handlers = {
  ping: () => "pong",
  time: () => Date.now(),
};
const result = (handlers[cmd] ?? (() => "noop"))();

// switch (true) pattern (use sparingly)
switch (true) {
  case score >= 90:
    return "A";
  case score >= 80:
    return "B";
  default:
    return "C";
}

// Exhaustiveness helper for unions (manual)
function assertNever(x) {
  throw new Error(`unexpected: ${x}`);
}

⚠️ Pitfalls

Missing break causes accidental fall-through bugs.
case values are compared with === — "1" does not match 1.
let/const in a case without braces collide with other cases.
Large switches are harder to test — prefer strategy maps for open sets of commands.
default is optional but recommended for unexpected values.

boolean.md — conditions
objects.md — dispatch tables
map.md — Map-based dispatch
error.md — default throws
oop.md — polymorphic alternatives

switch / case

switch / case

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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