StrictNullChecks

StrictNullChecks is a TypeScript compiler option that affects how the language handles null and undefined. When enabled, null and undefined have their own distinct types and values of other types cannot include them unless explicitly allowed. This increases type safety by preventing common runtime errors caused by null references.

Under strictNullChecks, variables declared with a non-nullable type such as string or number cannot be assigned null or undefined. To represent a value that might be absent, you must use union types like string | null or T | undefined, or perform runtime checks before use. Type narrowing through control flow analysis, such as if (x != null) { ... }, helps the compiler refine types. Features like optional chaining (a?.b) and nullish coalescing (a ?? b) are designed to work with this mode.

Enabling: In a tsconfig.json file, set "strictNullChecks": true or enable the broader "strict" mode which turns on a set of checks including strictNullChecks. If you enable it in existing code, you may need to annotate parameters and return types with union types and insert null checks.

Impact and usage: The option is widely adopted for safer codebases. It complements other TypeScript checks and encourages explicit handling of missing values. For developers migrating from non-strict configurations, using helper functions or runtime guards helps transition smoothly, and modern language features like optional chaining and nullish coalescing can simplify code while preserving safety.