Literal Inference

Literal Inference is a feature in TypeScript that allows the type of a variable or parameter to be inferred based on its value.

In the following example we can see that TypeScript considers x a literal type as the value cannot be changed any time later, when instead y is inferred as string as it can be modified any time later.

const x = 'x'; // Literal type of 'x', because this value cannot be changed
let y = 'y'; // Type string, as we can change this value

In the following example we can see that o.x was inferred as a string (and not a literal of a) as TypeScript considers that the value can be changed any time later.

type X = 'a' | 'b';

let o = {
    x: 'a', // This is a wider string
};

const fn = (x: X) => `${x}-foo`;

console.log(fn(o.x)); // Argument of type 'string' is not assignable to parameter of type 'X'

As you can see the code throws an error when passing o.x to fn as X is a narrower type.

We can solve this issue by using type assertion using const or the X type:

let o = {
    x: 'a' as const,
};

or:

let o = {
    x: 'a' as X,
};