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What are literal types?

Q: What are literal types?

**Target Audience:** Beginners who know basic TypeScript types and want to model "only these exact values". **Difficulty:** Easy **TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`). > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Every compiler message and every output shown was produced by actually running the code; the **Verified** section quotes it in full. ## 1. Why This Even Matters — A Story First A coffee machine has four buttons: espresso, latte, tea, hot water. There is no button that says "whatever drink you type in". If you could type any text, someone would eventually type "lattee" and the machine would do something strange. Typing a parameter as `string` is the "type anything" keyboard. A **literal type** is the row of buttons: the checker only accepts the exact values you listed, and a typo like `"lattee"` i

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11 of 81

The problem

Question presented to candidate: "What is a literal type in TypeScript? Show me where you would use one, and explain why `const x = 'up'` and `let x = 'up'` end up with different types."

What a strong answer should cover:

A literal type is a type with exactly one value (`"up"`, `42`, `true`); a union of literals models a closed set of allowed values
`boolean` is just `true | false`; string, number, boolean and bigint literals all exist
Widening: `const` keeps the literal type, `let` and object-literal properties widen to `string` / `number`, which is the classic "string is not assignable to 'GET' | 'POST'" error
The fixes: annotate with the union, use `as const`, or derive the union from a const array with `(typeof ARR)[number]`
Comparing against a literal (`===`, `switch`) narrows the union, enabling exhaustiveness checks with `never`
Literal types are erased: untrusted strings still need a run-time guard

Code / implementation expected: Yes — a function that accepts a literal union, a widening error and its fix.

literal-types

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Solution

Target Audience: Beginners who know basic TypeScript types and want to model "only these exact values". Difficulty: Easy TypeScript version: Compiler behaviour checked with TypeScript 5.4.5 (tsc --strict).

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. Every compiler message and every output shown was produced by actually running the code; the Verified section quotes it in full.

1. Why This Even Matters — A Story First

A coffee machine has four buttons: espresso, latte, tea, hot water. There is no button that says "whatever drink you type in". If you could type any text, someone would eventually type "lattee" and the machine would do something strange.

Typing a parameter as string is the "type anything" keyboard. A literal type is th

The solution is hidden so you can try first

9 min read · 10 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

TypeScriptLiteral types, literal unions and const vs let

Practice

TypeScriptObject widening, as const, deriving a union and narrowing

Practice

Discussion

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