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What are the basic types in TypeScript?

Q: What are the basic types in TypeScript?

**Target Audience:** Beginners writing their first TypeScript annotations. **Difficulty:** Easy **TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022). > **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. All output shown was produced by actually running the code; the **Verified** section quotes it in full. ## 1. Why This Even Matters — A Story First Think of a post office sorting room. Every parcel goes into a bin by **kind**: letters, small boxes, large boxes, fragile items. Once a bin is labelled "letters", a sorter who tries to drop a bicycle into it gets stopped immediately. A type is that bin label. When you write `let port: number`, you are labelling the bin, and TypeScript stops anyone putting the string `"8080"` into it. To label bins you need a vocabulary of kinds. TypeScript's basic

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The problem

Question presented to candidate: "Walk me through the basic types in TypeScript. Which ones map to JavaScript values, and what are the special ones like any, unknown, void and never for?"

What a strong answer should cover:

The seven primitives, lowercase: string, number, bigint, boolean, symbol, null, undefined (and why String is the wrong type)
Arrays (number[] / Array<number>), tuples ([string, number]) and object types with optional properties
The special types: any disables checking, unknown must be narrowed, void means no useful return, never means no possible value
unknown as the top type and never as the bottom type, with any outside the ordering
Classic traps: {} accepts 5, typeof null is "object", bigint does not mix with number, and types are erased at run time

Code / implementation expected: Optional — a few annotated declarations and one rejected assignment.

types
basics

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Solution

Target Audience: Beginners writing their first TypeScript annotations. Difficulty: Easy TypeScript version: Every compiler message and output below was produced with TypeScript 5.4.5 (--strict, target ES2022).

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. All output shown was produced by actually running the code; the Verified section quotes it in full.

1. Why This Even Matters — A Story First

Think of a post office sorting room. Every parcel goes into a bin by kind: letters, small boxes, large boxes, fragile items. Once a bin is labelled "letters", a sorter who tries to drop a bicycle into it gets stopped immediately. A type is that bin label. When you write let port: number, you are labelling the bin, and TypeScript stops anyone pu

The solution is hidden so you can try first

9 min read · 9 sections

Run the code

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

TypeScriptPrimitives, arrays, tuples and object types

Practice

TypeScriptThe special types: any, unknown, void, never and literals

Practice

Discussion

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