TypeScript determine type by running compile-time function-CodePudding

Let's say I have the following piece of code :

const value = JSON.parse('{"foo": "bar"}');

Here value has type any because of the JSON.parse signature.

Considering the JSON.parse function only has hard-coded argument(s) and has no side effects, it could be run successfully at compile time. Can I ask TypeScript to evaluate this function at compile time, so it can provide me strong typing for the value variable ?

CodePudding user response：

Someone implemented once a JSON parser in typescript I saw it here.

By taking his implementation of ParseJson you can do something like this

function parseWithTypes<T extends string>(json: T) {
  return JSON.parse(json) as ParseJson<T>;
}

const value = parseWithTypes('{"foo": "bar"}');

Playground

One caveat that it has is that it will infer the types as constant, meaning that the type foo won't be string but 'bar' instead.

CodePudding user response：

Do you mean this?

interface Foo {
    bar: string;
}

const value = <Foo>JSON.parse('{"foo": "bar"}');
const value1 = JSON.parse('{"foo": "bar"}') as Foo;