How do enums work and when should you use them?

An enum creates a runtime object whose members are constants. Numeric members are auto-incremented and also produce a reverse mapping from value to name. String members must be initialized explicitly and do not get a reverse mapping, which makes them easier to read in logs and API payloads.

enum Status {
  Idle = "idle",
  Loading = "loading",
  Error = "error",
}

function render(status: Status): string {
  return "State: " + status;
}

render(Status.Loading);

Runtime behavior and trade-offs

• Enums emit a JavaScript object, so they add runtime code
• Numeric enums allow reverse lookups, which can be surprising
• String enums are explicit and serialize predictably
• Const enums are inlined but have limitations with isolatedModules and cross-package use
• A union of string literals erases completely and narrows naturally

For application state, a literal union is often the better default: it has no runtime cost, works with discriminated unions and is easy to derive from an API response. Enums are reasonable when a stable runtime object is genuinely useful or when a public API already exposes one.