What does the 'strict' flag in tsconfig enable? | Interview Question

Q: What does the 'strict' flag in tsconfig enable?

**Target Audience:** Developers who have `"strict": true` in their config and want to know exactly what it switched on. **Difficulty:** Medium **TypeScript version:** The flag list and every error below come from TypeScript 5.4.5 (its option table and real `tsc` runs); the 5.6 and 6.0 changes are cited from 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 compiler message and output was produced by actually running the code; the **Verified** section quotes it in full. ## 1. Why This Even Matters — A Story First A building inspector can be booked for a "basic" visit or a "full" one. The basic visit checks that walls are standing. The full visit also checks that every socket is earthed, every exit is marked, every fire door closes. Same building, same price of bricks — but only the full visit catches the wiring fault

The problem

Question presented to candidate: "Most projects set strict: true in tsconfig. What does that one flag actually turn on, and what kinds of bugs does each part catch?"

What a strong answer should cover:

strict is an umbrella for the strict family; each member can be overridden individually
The members in 5.4: noImplicitAny, strictNullChecks, strictFunctionTypes, strictBindCallApply, strictPropertyInitialization, noImplicitThis, alwaysStrict, useUnknownInCatchVariables, plus strictBuiltinIteratorReturn added in 5.6
Why strictNullChecks and noImplicitAny matter most, with a concrete crash each one prevents
Details: method-syntax parameters stay bivariant, strictPropertyInitialization needs strictNullChecks, catch variables become unknown
What is not included (noUncheckedIndexedAccess, exactOptionalPropertyTypes) and how to roll strict out on an existing codebase

Code / implementation expected: Optional — one or two snippets that compile without strict and fail with it.

config
tsconfig

Try it yourself

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

Solution

Target Audience: Developers who have "strict": true in their config and want to know exactly what it switched on. Difficulty: Medium TypeScript version: The flag list and every error below come from TypeScript 5.4.5 (its option table and real tsc runs); the 5.6 and 6.0 changes are cited from 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 compiler message and output was produced by actually running the code; the Verified section quotes it in full.

1. Why This Even Matters — A Story First

A building inspector can be booked for a "basic" visit or a "full" one. The basic visit checks that walls are standing. The full visit also checks that every socket is earthed, every exit is marked, every fire

The solution is hidden so you can try first

11 min read · 10 sections

Run the code

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

TypeScriptnoImplicitAny, strictNullChecks and unknown catch variables

Practice

TypeScriptFunction, bind/call/apply, this and class checks

Practice

Discussion

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