CODEWITHSUNDEEP
javascripttypescript
Dušan Novák
Dušan Novák

Reputation: 83

Typescript compiler does not check validity of method' return type

Here is my problem: I have a class which implements interface. Method greet should have return type void but in my implemented class it has string and compiler doesn't warn me. And IDE neither (I'm using PhpStorm). Am I missing something or is this intentional?

interface Person {
    sex: string;
    greet() : void;
}

class Boy implements Person {
    sex: 'M';
    greet() {
        return this.sex;
    }
}

I'm using Typescript 2.0.10

Upvotes: 1

Views: 371

Answers (1)

Nitzan Tomer
Nitzan Tomer

Reputation: 164139

It won't complain because it doesn't really matter.
Your interface declares that the method returns void, if your implementation returns a value then no harm will happen.
Example:

let person: Person = new Boy();
person.greet();

As person is of type Person (and not Boy) then the greet method won't return a value, and indeed I'm not trying to use a return value.

On the other hand if it was the opposite:

interface Person {
    sex: string;
    greet(): string;
}

class Boy implements Person {
    sex: 'M';
    greet(): void {}
}

Then an error is thrown:

Class 'Boy' incorrectly implements interface 'Person'.
Types of property 'greet' are incompatible.
Type '() => void' is not assignable to type '() => string'.
Type 'void' is not assignable to type 'string'.

Upvotes: 2

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