49 lines
1.4 KiB
TypeScript
49 lines
1.4 KiB
TypeScript
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export function assertNever(value: never, errorMessage?: string): never
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export function assertNever(value: never, getErrorMessage?: (value: unknown) => string): never
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export function assertNever(value: never, noThrow?: boolean): never
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/**
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* Helper function for exhaustive checks of discriminated unions.
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* https://basarat.gitbooks.io/typescript/docs/types/discriminated-unions.html
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*
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* @example
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*
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* type A = {type: 'a'};
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* type B = {type: 'b'};
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* type Union = A | B;
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*
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* function doSomething(arg: Union) {
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* if (arg.type === 'a') {
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* return something;
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* }
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*
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* if (arg.type === 'b') {
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* return somethingElse;
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* }
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*
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* // TS will error if there are other types in the union
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* // Will throw an Error when called at runtime.
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* // Use `assertNever(arg, true)` instead to fail silently.
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* return assertNever(arg);
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* }
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*/
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export function assertNever(value: never, errorMessageOrNoThrow?: string | ((value: unknown) => string) | boolean): never {
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if (typeof errorMessageOrNoThrow === 'string') {
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throw new Error(errorMessageOrNoThrow)
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}
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if (typeof errorMessageOrNoThrow === 'function') {
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throw new Error(errorMessageOrNoThrow(value))
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}
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if (errorMessageOrNoThrow) {
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return value
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}
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throw new Error(
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`Unhandled discriminated union member: ${JSON.stringify(value)}`,
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);
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}
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export default assertNever;
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