带有duck-typed对象的Typescript字符串文字

Question

Typescript 1.8 introduced the string literal type. However, when passing in an object as a parameter as in the following:

const test = {
    a: "hi",
    b: "hi",
    c: "hi"
};

interface ITest {
    a: "hi" | "bye"
}

function testFunc (t: ITest) {

}

testFunc(test);

It fails with:

Argument of type '{ a: string; b: string; c: string; }' is not assignable to parameter of type 'ITest'. Types of property 'a' are incompatible. Type 'string' is not assignable to type '"hi" | "bye"'. Type 'string' is not assignable to type '"bye"'.

I would expect this to work since it meets the requirements of the interface, but I might be overlooking something.

Answer 1

The type of test.a has been inferred as string and not "hi" . The compiler is comparing the types and not the initial string expression.

In order to make this work you need to type that property as "hi" | "bye" "hi" | "bye" :

type HiBye = "hi" | "bye";

const test = {
    a: "hi" as HiBye,
    b: "hi",
    c: "hi"
};

interface ITest {
    a: HiBye
}

function testFunc (t: ITest) {
}

testFunc(test);

Note that in the original case it wouldn't make sense for the compiler to infer the type of test.a to be "hi" because you can assign a different value to test.a before it reaches testFunc(test) —ex. test.a = "not hi" .

Side note: It's good the compiler doesn't infer the type the be the string expression for even constant string variables. That would also lead to a lot of annoyances... imagine this:

const myVariableTypedAsHi = "hi";   // implicitly typed as "hi"
let otherVar = myVariableTypedAsHi; // otherVar implicitly typed as "hi"

otherVar = "test"; // error: cannot assign `"test"` to `"hi"`—well that would be annoying