Disallow truthiness coercions on known-true/known-false values · Issue #9041 · microsoft/TypeScript (original) (raw)

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Description

Inspired by #7746 but making a new issue for clarity

Bad code:

function func(): boolean { return Math.random() > 0.5; } if (func) { // oops, meant to write func() }

Change: Under --strictNullChecks, it becomes an error to use an expression in a truthiness position unless the type of the expression is possibly-falsy

Possibly-falsy types are:

• any
• The primitives: string, string literal types, number, enum types, and boolean
• Type parameters
• Union types containing possibly-falsy types
• An intersection type where any member is possibly falsy

A truthiness position is:

• The test expression of an if, while, or do/while
• The middle expression of a for
• The first operand of ?, && or ||
• The operand of !