I'm using Typescript 3.3.
There is a type from a library: type Fn = (arg0: t1...) => any,
and I imported it, want to use it to sign an instance method:
class A {
mtd() {} // to be signed
}
How should I do?
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1 Answer
If your intention is to reuse the type signature of Fn in order to avoid repetition, you can do that, but the method will depend on whether mtd must be a proper method or an arrow function assigned to a class property.
Class methods and function properties are not the same. They differ in terms of handling
this, which makes property functions incompatible with inheritance. Proceed with caution.
Class method
type Fn = (argument: string) => any;
interface IA {
mtd: Fn;
}
class A implements IA {
/**
* Parameter types must be repeated 😞 but they will be type-checked as expected.
*/
mtd(argument: string) {
/* ... */
}
}
Arrow function property
type Fn = (argument: string) => any;
class A {
/**
* We know `argument` is a `string.
*/
mtd: Fn = argument => {
/* ... */
}
}
2 Comments
jcalz
This answer changes the method on the class prototype to a separate function-valued property on each instance... and as an arrow function it also loses the
this context. That might be acceptable, but the difference should probably be noted.
Karol Majewski
@jcalz you're right, the difference should be made explicit. I have updated my answer. Thank you for noticing that.