EasyFrontend9 min read

What is the difference between any and unknown?

Q: What is the difference between any and unknown?

**Target Audience:** Beginners who have seen `any` everywhere and want to know why reviewers keep asking for `unknown`. **Difficulty:** Easy **TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`); the history of `unknown` checked against the release notes linked inline. > **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 parcel arrives with no label. Two ways to deal with it: - **The `any` way:** assume it is whatever you hoped for. "It is probably the new phone" — you plug it in, and it turns out to be a jar of honey. - **The `unknown` way:** accept the parcel, but open it and look before you use it. Shake it, read the contents, then decide. Both let

759 views

5 of 81

The problem

Question presented to candidate: "Both `any` and `unknown` can hold any value. So what is the difference, and which one would you use for the result of `JSON.parse` or for a caught error?"

What a strong answer should cover:

Both are top types for assignment: every value can be assigned to either
`any` disables checking: every property access, call and assignment compiles, and it is contagious (whatever you read from it is `any` too)
`unknown` is the safe top type (added in TypeScript 3.0): you must narrow it with `typeof`, `instanceof`, `Array.isArray`, `in` or a type guard before using it, and it is assignable only to `unknown` and `any`
Practical uses: `unknown` for untrusted input (`JSON.parse`, network data, catch variables under `strict`), `any` only as a temporary migration escape hatch
Type algebra: `unknown | T` is `unknown`, `unknown & T` is `T`, `keyof unknown` is `never`

Code / implementation expected: Yes — the same operation on `any` and `unknown`, and a narrowing function that accepts `unknown`.

any
unknown

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners who have seen any everywhere and want to know why reviewers keep asking for unknown. Difficulty: Easy TypeScript version: Compiler behaviour checked with TypeScript 5.4.5 (tsc --strict); the history of unknown checked against the release notes linked inline.

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 parcel arrives with no label. Two ways to deal with it:

The any way: assume it is whatever you hoped for. "It is probably the new phone" — you plug it in, and it turns out to be a jar of honey.
The unknown way: accept the

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.

TypeScriptany compiles anything, unknown must be narrowed

Practice

TypeScriptType algebra, contagious any, JSON.parse and catch variables

Practice

Discussion

Other approaches, follow-ups and how it went in real interviews.

No comments yet. Share how you would answer it.

Question 5 of 81 in TypeScript

Keep your momentum going.

Mark this question solved to track your progress, then take the next one while it is fresh.

All TypeScript questions

Up nextEasy→

What are the basic types in TypeScript?

Continue