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39 lines
1.2 KiB
39 lines
1.2 KiB
/**
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* TypeScript will infer a string union type from the literal values passed to
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* this function. Without `extends string`, it would instead generalize them
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* to the common string type.
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*
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* Example definition:
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* const Race = StringUnion(
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* "orc",
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* "human",
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* "night elf",
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* "undead",
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* );
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* type Race = typeof Race.type;
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*
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* For more details, see:
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* https://stackoverflow.com/questions/36836011/checking-validity-of-string
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* -literal-union-type-at-runtime?answertab=active#tab-top
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*/
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export const StringUnion = <UnionType extends string>(...values: UnionType[]) => {
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Object.freeze(values);
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const valueSet: Set<string> = new Set(values);
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const guard = (value: string): value is UnionType => {
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return valueSet.has(value);
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};
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const check = (value: string): UnionType => {
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if (!guard(value)) {
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const actual = JSON.stringify(value);
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const expected = values.map(s => JSON.stringify(s)).join(' | ');
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throw new TypeError(`Value '${actual}' is not assignable to type '${expected}'.`);
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}
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return value;
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};
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const unionNamespace = {guard, check, values};
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return Object.freeze(unionNamespace as typeof unionNamespace & {type: UnionType});
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};
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