How can I define a Typescript interface for a function?

Question

I am defining the following Typescript interface. clickCustomButton1 should return nothing but I am not sure how to specify that.

interface IButtonTemplate {
    clickCustomButton1: (); // How can I say this should return nothing?
    // more code here
}

I use this in my code like this:

clickCustomButton1: null

then later:

newTopicTests = () => {
}

clickCustomButton1 = this.newTopicTests();

It's giving me an error saying:

Error   2   Cannot convert 'void' to '() => boolean'

Can someone give me an idea what I am doing wrong? What I think I need to do is to specify that clickCustomButton1 and also the newTopicTests do not return anything. But how can I do that with Typescript?

user2864740 - I am sorry but I am not sure what I understand what you mean. I added a line to my question asking if someone can show me how I can do this and make it work when anything I assign to clickCustomButton1 should meet the criteria that it return nothing. — Samantha J T Star
– Samantha J T Star, Commented Sep 26, 2014 at 10:14

user2864740 · Accepted Answer · 2014-09-26 10:31:44Z

2

The problem is because the lambda () => {} is typed as (): void because it returns nothing and thus has no [other] type inferred.

Thus, given f = () => {}, the expression f() is also typed as void - but clickCustomButton1 must return a boolean as it is declared.

Compare when using the following lambda, which is typed as (): boolean, which is now type-valid:

newTopicTests = () => true

Another way to see this problem is to write the original code as:

newTopicTests = (): boolean => {}

(This will also fail to compile, but will show the error closer to where it the source.)

After the updates to the question..

To declare a method in an Interface to return nothing, use:

clickCustomButton1(): void;

To declare a member which has the type of (): void, use

clickCustomButton1: () => void;

Also note that null is something whereas void represents nothing.

edited Sep 26, 2014 at 10:31

answered Sep 26, 2014 at 10:13

user2864740

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