CODEWITHSUNDEEP
typescript
Socrates
Socrates

Reputation: 9604

TypeScript with empty function as parameter?

I have a piece of code that I am having a hard time to understand. I'm new to TypeScript.

registerCommands(registry: CommandRegistry): void {
    registry.registerCommand(FensterCommands.HELLOWORLD);
    registry.registerHandler(FensterCommands.HELLOWORLD.id, {
        isEnabled: () => true,
        execute: () => this.messageService.info('Hello World!')
    });
}

Could someone help me understand the above code? The thing I don't understand is the second JSON-like parameter within registry.registerHandler(...). What type does this parameter value have, that is located within curly brackets {...}? The first parameter has the name isEnabled, right? And its value is what? Is it () or true? What does the empty function () mean? Does the entire line () => true end in being a comparison that ends in true/false?

Is that more or less correct?

Upvotes: 5

Views: 17019

Answers (2)

codeteq
codeteq

Reputation: 1532

What type the second paramter of registry.registerHandler is, depends on the definition of the function, it is an object but the informations you provided are not clear enough - it could be an interface or even any...

Despite this:

   isEnabled: () => true,

likely sets isEnabled to a function without parameter which returns a boolean value

  execute: () => this.messageService.info('Hello World!')

likely sets execute to a function without parameter which will return the type of this.messageService.info('Hello World!') returns.

Both functions are declared with ES6 Arrow functions

It could be also written as:

(Assuming this.messageService.info('Hello World!') will return nothing/void)

{
       isEnabled: function() { return true; },
       execute: function() { return this.messageService.info('Hello World!'); }
}

Upvotes: 7

Titian Cernicova-Dragomir
Titian Cernicova-Dragomir

Reputation: 250346

The second parameter is an object literal with the fields isEnabled and execute. You might be able to pass an object literal with more fields, without the definition of registerHandler it's impossible to tell.

As for the () => true that is an arrow function, a function with no parameters () that returns true. So the type of isEnabled is a function that returns a boolean and takes no parameters.

The execute field is similar, it is a function with no parameters and the body this.messageService.info('Hello World!')

Upvotes: 3

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