Type guard function does not work for string literal types · Issue #7441 · microsoft/TypeScript (original) (raw)

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Description

TypeScript Version:

1.8.2

Code

type Click = 'click';

function isClick(event: string): event is Click { return event === 'click'; }

function validateClickEvent(event: string): Click { if (isClick(event)) { return event; // this line raises a compilation error: Type 'string' is not assignable to type '"click"' } throw new Error('not click'); }

type Creature = {}; type Animal = {} & Creature;

function isAnimal(creature: Creature): creature is Animal { return true; }

function validateAnimalCreature(creature: Creature): Animal { if (isAnimal(creature)) { return creature; // this line does not raise any compilation error } throw new Error('not animal'); }

Expected behavior:

Can apply type guard to string literal types.

Actual behavior:

Cannot apply type guard to string literal types.