TypeScript Strict Mode : cinq pannes runtime evitees

TypeScript Strict Mode n est pas une preference de style. Il deplace vers l editeur les erreurs de production: valeurs absentes, any qui se propage, index non sur, contrats de fonctions faibles et JSON non valide. Les donnees externes doivent entrer comme unknown, etre verifiees a la frontiere, puis devenir un modele interne stable.

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1. null et undefined

TypeScript Strict Mode n est pas une preference de style. Il deplace vers l editeur les erreurs de production: valeurs absentes, any qui se propage, index non sur, contrats de fonctions faibles et JSON non valide. Les donnees externes doivent entrer comme unknown, etre verifiees a la frontiere, puis devenir un modele interne stable. The practical rule is to decide behavior before runtime: empty state, fallback value, controlled error, or validated model. This keeps reviews focused and prevents hidden assumptions from reaching users.

2. propagation de any

TypeScript Strict Mode n est pas une preference de style. Il deplace vers l editeur les erreurs de production: valeurs absentes, any qui se propage, index non sur, contrats de fonctions faibles et JSON non valide. Les donnees externes doivent entrer comme unknown, etre verifiees a la frontiere, puis devenir un modele interne stable. The practical rule is to decide behavior before runtime: empty state, fallback value, controlled error, or validated model. This keeps reviews focused and prevents hidden assumptions from reaching users.

3. index non sur

TypeScript Strict Mode n est pas une preference de style. Il deplace vers l editeur les erreurs de production: valeurs absentes, any qui se propage, index non sur, contrats de fonctions faibles et JSON non valide. Les donnees externes doivent entrer comme unknown, etre verifiees a la frontiere, puis devenir un modele interne stable. The practical rule is to decide behavior before runtime: empty state, fallback value, controlled error, or validated model. This keeps reviews focused and prevents hidden assumptions from reaching users.

4. contrats faibles

TypeScript Strict Mode n est pas une preference de style. Il deplace vers l editeur les erreurs de production: valeurs absentes, any qui se propage, index non sur, contrats de fonctions faibles et JSON non valide. Les donnees externes doivent entrer comme unknown, etre verifiees a la frontiere, puis devenir un modele interne stable. The practical rule is to decide behavior before runtime: empty state, fallback value, controlled error, or validated model. This keeps reviews focused and prevents hidden assumptions from reaching users.

5. JSON non valide

TypeScript Strict Mode n est pas une preference de style. Il deplace vers l editeur les erreurs de production: valeurs absentes, any qui se propage, index non sur, contrats de fonctions faibles et JSON non valide. Les donnees externes doivent entrer comme unknown, etre verifiees a la frontiere, puis devenir un modele interne stable. The practical rule is to decide behavior before runtime: empty state, fallback value, controlled error, or validated model. This keeps reviews focused and prevents hidden assumptions from reaching users.

Rollout checklist

• Enable strict in a branch and group errors by risk.
• Start with null handling and external data boundaries.
• Replace implicit any with unknown plus validation.
• Add explicit return types to public functions.
• Keep temporary casts visible until they are removed.

FAQ

Does Strict Mode slow development?

During migration it can, but it reduces production rework by exposing shape and contract errors earlier.

Is optional chaining enough?

No. It prevents one crash but does not decide the user experience when data is missing.

Do schema libraries matter?

Small boundaries can use handwritten guards. Larger products benefit from shared schemas.