29 lines
799 B
TypeScript
29 lines
799 B
TypeScript
// @strict: true
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// Using a homomorphic mapped type over `T`
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// Produces a lower-priority inference for `T` than other
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// positions, allowing one to override the priority the argument
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// order would usually imply
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type Lower<T> = { [K in keyof T]: T[K] };
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export function appendToOptionalArray<
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K extends string | number | symbol,
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T
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>(
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object: { [x in K]?: Lower<T>[] },
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key: K,
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value: T
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) {
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const array = object[key];
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if (array) {
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array.push(value);
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} else {
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object[key] = [value];
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}
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}
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// e.g.
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const foo: {x?: number[]; y?: string[]; } = {};
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appendToOptionalArray(foo, 'x', 123); // ok
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appendToOptionalArray(foo, 'y', 'bar'); // ok
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appendToOptionalArray(foo, 'y', 12); // should fail
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appendToOptionalArray(foo, 'x', "no"); // should fail
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