CODEWITHSUNDEEP
typescript
Max Koretskyi
Max Koretskyi

Reputation: 105517

What's the difference let f:F = f` and `let f = <F>f`

I have the following code:

interface F {
    (): string;
    a(): number;
}

function f() {
    return '3';
}

f['a'] = function () {
    return 3;
};

Then I want to assign a function to a variable. I can do it like this:

let z = <F>f; // works

or like this:

let y: F = f; // doesn't work

What's the difference?

Upvotes: 3

Views: 56

Answers (2)

Nitzan Tomer
Nitzan Tomer

Reputation: 164287

This is how I would do it:

interface F {
    (): string;
    a(): number;
}

function f() {
    return '3';
}

let f1 = f as F;
f1.a = function () {
    return 3;
};

(code in playground)

Using as F is equivalent to <F> and it's called type assertion.

Edit

Yes, you can define f to be of type F like so:

let f3 = function() {
    return "3";
} as F;

f.a = function () {
    return 3;
};

Or using an arrow function:

let f = (() => {
    return "3";
}) as F;

Upvotes: 1

T.J. Crowder
T.J. Crowder

Reputation: 1074959

The root issue is that f is just a function, and you're trying to use it as though it were an instance of an interface. (And I think that's fine, it's compatible with the interface, it's just a syntax thing.)

This is fine:

let z = <F>f; // works

...because it's using a cast to tell TypeScript that although f is just a function as far as TypeScript knows, you know better and it's compatible with the interface F. Then type inference comes into play and assigns the type F to z because the right-hand side of the assignment is of type F.

But this:

let y: F = f; // doesn't work

...doesn't work because it's declaring y as being type F, and then assigning a function to it. The function isn't of type F, so the assignment fails.

Upvotes: 3

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