Type inference for constant objects with string literal types

Hi!

Let's say that I have this code:

interface Greeter {
    hello: 'world'
}

function hello() {
    const first: Greeter = {
        hello: 'world'
    };

    const obj = {
        hello: 'world'
    };

    const second: Greeter = obj;
}

For the first object the compiler can infer type of hello as a string literal type and outputs no errors. However, for the second object can't infer this, although obj is a constant object, and throws an error.

It would be nice to have automatic inference here. For example, in Flow types are inferred here automatically and the same code passes type checks without errors.

Addition: specifying the string type literal explicitly helps:

const obj = {
    hello: 'world' as 'world'
}

but makes the code look verbose.

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[MartinJohns]

However, for the second object can't infer this, although obj is a constant object, and throws an error.

The variable obj is constant, but the object is not. You can re-assign a new value with obj.hello = "fubar"; just fine.

What you want can be achieve with the "const contexts" feature introduced in TypeScript 3.4: #29510

const obj = { hello: 'world' } as const;

[x] This wouldn't be a breaking change in existing TypeScript/JavaScript code

This would definitely be a breaking change in existing code. If it would infer obj.hello as the type 'world', then re-assigning a different string would not work anymore.

What you want can be achieve with the "const contexts" feature introduced in TypeScript 3.4: #29510

Yes, "const contexts" make the code look better here, I didn't know about them.

This would definitely be a breaking change in existing code. If it would infer obj.hello as the type 'world', then re-assigning a different string would not work anymore.

I agree, so what I was actually thinking about was more similar to automatic inference of "const context" in some cases. In my example "obj" cannot be changed from the outside of the function and it is not changed inside, so in this particular case the compiler could infer that obj.hello can never change.

[MartinJohns]

I was actually thinking about was more similar to automatic inference of "const context" in some cases.

I see. Personally I think it's really not worth the additional code (complexity) and performance cost (the analysis), when we already have a very good feature to solve this without hassle: const contexts.

[typescript-bot]

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