How does TypeScript handle null and undefined safely?

Q: How does TypeScript handle null and undefined safely?

**Target Audience:** Beginners who have met "Cannot read properties of undefined" and want the compiler to catch it first. **Difficulty:** Easy **TypeScript version:** Compiler behaviour checked with TypeScript 5.4.5 (`tsc --strict`); version history 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 Tony Hoare, who invented the null reference in 1965, later called it his "billion-dollar mistake" (in a 2009 QCon London talk, [Null References: The Billion Dollar Mistake](https://qconlondon.com/london-2009/qconlondon.com/london-2009/presentation/Null%2BReferences_%2BThe%2BBillion%2BDollar%2BMistake.html)). The trouble is not that "no value" ex

The problem

Question presented to candidate: "Null reference errors are one of the most common JavaScript crashes. How does TypeScript help you avoid them, and which operators and patterns do you use day to day to handle values that might be missing?"

What a strong answer should cover:

`strictNullChecks` (part of `strict`, since TypeScript 2.0) takes `null` and `undefined` out of every type, so "may be missing" must be written as `T | null` or `T | undefined`
Narrowing removes them: `!== null`, `== null` (matches both null and undefined), truthiness checks (and their trap with `0` and `""`), early returns
Operators: `??` falls back only on null/undefined, unlike `||`; `?.` short-circuits to `undefined`
Library signatures that can miss: `Array.prototype.find`, `Map.get`, and `NonNullable<T>`
The non-null assertion `!` only silences the compiler and does no run-time check
Narrowing limits: callbacks over a reassigned `let`, and `filter` needing an explicit type predicate before TypeScript 5.5

Code / implementation expected: Yes — a function returning `T | null`, the compiler error when it is used unchecked, and the `??` versus `||` difference.

optional
nullish

Try it yourself

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

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

Solution

Target Audience: Beginners who have met "Cannot read properties of undefined" and want the compiler to catch it first. Difficulty: Easy TypeScript version: Compiler behaviour checked with TypeScript 5.4.5 (tsc --strict); version history 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

Tony Hoare, who invented the null reference in 1965, later called it his "billion-dollar mistake" (in a 2009 QCon London talk, [Null References: The Billion Dollar Mistake](https://qconlondon.com/london-2009/qconlondon.com/london-2009/presentation

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10 min read · 10 sections

Run the code

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

TypeScriptstrictNullChecks, narrowing, ?? versus || and the ! trap

Practice

TypeScriptLookups that can miss, NonNullable, filter and closures

Practice

Discussion

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