← All notes

Answer: You can then turn individual checks off. New TypeScript versions may add checks under this flag, so upgrades can surface new errors.

{ "compilerOptions": { "strict": true } }

What strict covers

• strictNullChecks
• noImplicitAny
• strictFunctionTypes
• strictBindCallApply
• strictPropertyInitialization
• noImplicitThis
• alwaysStrict
• useUnknownInCatchVariables

strictNullChecks matters most

When it is off, null and undefined are effectively ignored, a classic source of runtime crashes. When on, they are distinct types:

const user = users.find(u => u.id === id);
user.name;   // error: user is possibly undefined
user?.name;  // OK

Adopting on an existing codebase

1. Enable strict.

Claims

• id: C1 claim: The strict flag enables a wide range of type checking behavior. source: S1 evidence: "The strict flag enables a wide range of type checking behavior"
• id: C2 claim: You can turn off individual strict mode family checks. source: S1 evidence: "You can then turn off individual strict mode family checks as needed."
• id: C3 claim: Future versions may add stricter checking under this flag. source: S1 evidence: "Future versions of TypeScript may introduce additional stricter checking under this flag"
• id: C4 claim: When strictNullChecks is false, null and undefined are effectively ignored. source: S2 evidence: "When strictNullChecks is false, null and undefined are effectively ignored by the language."

Sources

• [S1] TypeScript: TSConfig Option: strict. TypeScript (Microsoft). https://www.typescriptlang.org/tsconfig/strict.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-04.
• [S2] TypeScript: TSConfig Option: strictNullChecks. TypeScript (Microsoft). https://www.typescriptlang.org/tsconfig/strictNullChecks.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_y43qgmdp,
  author = {{iinventedlight}},
  title = {{TypeScript strict mode}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/typescript-strict-mode/},
  note = {Permanent ID iil:note:y43qgmdp. License: CC-BY-4.0.},
}

Markdown

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For language models and agents: start at /llms.txt. This page in Markdown: /library/notes/typescript-strict-mode.md.