strict-type-predicates.md

Disallow always true (or false) type predicates (strict-type-predicates)

Warns for type predicates that are always true or always false. Works for typeof comparisons to constants (e.g. typeof foo === 'string'), and equality comparison to null/undefined. (TypeScript won’t let you compare 1 === 2, but it has an exception for 1 === undefined.) Does not yet work for instanceof. Does not warn for if (x.y) where x.y is always truthy. For that, see strict-boolean-expressions.

This rule requires strictNullChecks to be enabled.

Examples of incorrect code for this rule:

const numberOrNull: number | null = 0;
// Implicitly checks for `!== undefined`, which is always false.
if (numberOrNull != null) {
  return;
}

const numberOrUndefined: number | undefined = 0;
// Implicitly checks for `!== null`, which is always false.
if (numberOrNull != undefined) {
  return;
}

const number: number = 0;
// Always false.
if (typeof number === 'string') {
  return;
}

Examples of correct code for this rule:

const numberOrNull: number | null = 0;
if (numberOrNull !== null) {
  return;
}

const numberOrUndefined: number | undefined = 0;
if (numberOrNull !== undefined) {
  return;
}

const number: number = 0;
// Always false.
if (typeof number === 'number') {
  return;
}

Related To

• TSLint: strict-type-predicates