Narrowing typeof operator comparison by type #49584
Description
TypeScript Version: All
Search Terms: Non-literal
Code: Playground Link
declare const foo: string | number;
declare const bar: unknown;
if (typeof bar === typeof foo) {
bar // Currently: unknown, Proposed: string | number
}
declare const baz: "string" | "number";
if (typeof bar === baz) {
bar // Currently: unknown, Proposed: string | number
}Use Case
type Value = string | number | boolean | [defaultValue: unknown, ...choices: [unknown, ...unknown[]]];
class Options {
constructor(public options: Map<string, Value>) {}
get(key: string) { return this.options.get(key); }
set(key: string, value: unknown) {
const currentValue = this.options.get(key);
if (typeof key === "undefined") return;
if (Array.isArray(currentValue)) {
const [, ...choices] = currentValue;
if (choices.includes(value)) this.options.set(key, [value, ...choices]);
return;
}
if (typeof value === typeof currentValue) this.options.set(key, value); // Error
}
}