What is tsconfig.json and which options matter most?

Q: What is tsconfig.json and which options matter most?

**Target Audience:** Beginners who have just run `tsc --init` and are staring at a wall of options. **Difficulty:** Easy **TypeScript version:** All compiler runs below used TypeScript 5.4.5; changes in later releases (5.9, 6.0) are cited from their release coverage and were not run here. > **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 emitted file and compiler message shown came from a real `tsc` run; the **Verified** section quotes the example output in full. ## 1. Why This Even Matters — A Story First Before a film shoot, the director hands the crew a one-page brief: which scenes we are shooting, which camera and lens, what format the footage is delivered in, how strict continuity checking will be. Nobody re-explains this every morning. `tsconfig.json` is that brief for the TypeScript compiler. It answers four questions once, for the whole project: 1

The problem

Question presented to candidate: "What is tsconfig.json for, and if you were setting up a new project, which compiler options would you actually care about and why?"

What a strong answer should cover:

tsconfig.json marks the project root and is read by both tsc and the editor; include, exclude, files, extends, references
strict (always on) and extra checks such as noUncheckedIndexedAccess
The environment options and how they differ: target changes emitted syntax, lib only changes which APIs the checker knows, module and moduleResolution describe imports
Output options: outDir/rootDir, declaration, sourceMap, noEmit when a bundler builds the code, plus skipLibCheck for speed
Defaults change across versions (tsc --init in 5.9, compiler defaults in 6.0), so set key options explicitly and inspect with tsc --showConfig

Clarifying questions expected:

Is this an app built by a bundler, or code that tsc compiles for Node.js or a published library?
Which runtime and JavaScript version must the output support?

Code / implementation expected: Optional — a short tsconfig for a stated kind of project, with each option justified.

tsconfig
tooling

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 just run tsc --init and are staring at a wall of options. Difficulty: Easy TypeScript version: All compiler runs below used TypeScript 5.4.5; changes in later releases (5.9, 6.0) are cited from their release coverage and were not run here.

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 emitted file and compiler message shown came from a real tsc run; the Verified section quotes the example output in full.

1. Why This Even Matters — A Story First

Before a film shoot, the director hands the crew a one-page brief: which scenes we are shooting, which camera and lens, what format the footage is delivered in, how strict continuity checking will be. Nobody re-explains this every morning. tsconfig.json

The solution is hidden so you can try first

11 min read · 11 sections

Run the code

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