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Compile-Time Type Assertion Helpers

Assert type equality at build time with Equal, Expect, IsAny and tiny runtime helpers, catching type regressions during tsc.

TypescriptCli Tooling

Code

snippet
export type Equal<Left, Right> =
  (<Value>() => Value extends Left ? 1 : 2) extends <Value>() => Value extends Right ? 1 : 2
    ? true
    : false;

export type Expect<Condition extends true> = Condition;

export type IsAny<Value> = 0 extends 1 & Value ? true : false;

export function expectType<Expected>(value: Expected): Expected {
  return value;
}

export function expectAssignable<Expected>(value: Expected): void {
  void value;
}

export type TypeAssertions = [
  Expect<Equal<string, string>>,
  Expect<Equal<IsAny<unknown>, false>>,
  Expect<Equal<IsAny<any>, true>>,
  Expect<Equal<"a" | "b", "b" | "a">>,
  Expect<Equal<{ id: number }, { id: number }>>,
];

const user = expectType<{ id: number }>({ id: 1 });
console.log(user.id);
expectAssignable<string | number>("ok");
expectAssignable<string | number>(42);

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