Microsoft/TypeScript
Differing user-defined type guard and 'typeof' type guard behaviour when narrowing 'any'
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#6 015 ouverte le 9 déc. 2015
Effort: ModerateHelp WantedSuggestion
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Description
In the below code, using a typeof type guard and an equivalent (I thought) user-defined guard, only one error is produced.
var y: any;
// Built-in type guard
if (typeof y === "string") {
y.hello = true; // Correct error - 'hello' does not exist on type string
}
// Equivalent user-defined type guard
function f(x: any): x is string {
return typeof x === "string";
}
if (f(y)) {
y.hello = true; // No error with user-defined guard
}
Playground demo. Looks like user-defined type guards won't narrow from any, in any circumstances, as far as I can tell.