For instance:
interface Foo {
someProperty: Number
someMethod?: (str: string) => void
}
class Bar implements Foo {
someProperty = 42
someMethod (str) {
console.log(this.someProperty)
}
}
The str argument in someMethod() is clearly a string. However, the compiler throws an error:
test.tsx(8,15): error TS7006: Parameter 'str' implicitly has an 'any' type.
What mistake am I making?
Edit: tsc --version indicates version 2.8.3.
Why isn't this working?
There is no issue with your code. Unfortunately, TypeScript does not support this feature, as discussed in detail here.
To resolve the issue, include an explicit annotation in the class.
It appears that the str parameter passed to someMethod() is expected to be a string.
However, this assumption may not always hold true. There are various ways to define the parameter signature while still satisfying the interface requirements. For instance:
someMethod (s: string | number) {
or
someMethod (s: any) {
The use of the implements clause acts as a validation after the fact, ensuring that your class conforms to the interface. It does not directly alter the types specified for properties or methods within your class definition.
If you were to omit the implements clause, an implicit any type would apply. With the inclusion of the implements clause, there remains an implicit any presence, as it does not affect the declared types of class members.
Implementing interfaces can go beyond just using classes. By utilizing a factory function that generates and outputs an object, you can avoid explicitly typing method arguments:
interface Foo {
someProperty: number
someMethod?: (str: string) => void
}
function createBar(): Foo {
return {
someProperty: 42,
someMethod: (str) => {
const n = str.length; // The parameter is inferred as str: string
}
}
}
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