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Awaiting More FeedbackThis means we'd like to hear from more people who would be helped by this featureThis means we'd like to hear from more people who would be helped by this featureSuggestionAn idea for TypeScriptAn idea for TypeScript
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unknown type guard
Related: #24439 (comment), #25172
Suggestion
Currently, only a very limited set of type guards are able to narrow the new unknown
type:
- Array.isArray (because it is a typeguard for
arg is any[]
) and probably some more in the lib files - instanceof
- self-written typeguards
However to make working with unknown types less awkward, I'd like to see a couple of other constructs being able to narrow the unknown
type:
let x: unknown;
// Direct equality should narrow to the type we compare to
x === "1"; // should narrow x to string or the literal "1" type, similar for other types aswell
// All these should narrow x to {prop: any}
"prop" in x;
x.prop != null;
x.prop !== undefined;
typeof x.prop !== "undefined";
// typeof should work on properties of the unknown variable
typeof x.prop === "string"; // should narrow x to {prop: string}
Use Cases
Make unknown
easier to work with!
// before, very verbose!
const x: unknown = undefined!;
function hasProp1(x: any): x is {prop1: any} {
return "prop1" in x;
}
function hasProp2(x: any): x is {prop2: any} {
return "prop2" in x;
}
// imagine doing this for many more properties
if (hasProp1(x)) {
x.prop1;
if (hasProp2(x)) {
x.prop2;
}
}
// ===========
// after, much more concise and less overhead
const x: unknown = undefined!;
if ("prop1" in x) {
x.prop1;
if ("prop2" in x) {
x.prop2;
}
}
Checklist
My suggestion meets these guidelines:
- This wouldn't be a breaking change in existing TypeScript / JavaScript code
- This wouldn't change the runtime behavior of existing JavaScript code
- This could be implemented without emitting different JS based on the types of the expressions
- This isn't a runtime feature (e.g. new expression-level syntax)
liolick, ThomasdenH, Kinrany, donaldpipowitch, jakubwolny and 87 more
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Awaiting More FeedbackThis means we'd like to hear from more people who would be helped by this featureThis means we'd like to hear from more people who would be helped by this featureSuggestionAn idea for TypeScriptAn idea for TypeScript