ES6 / Typescript - Typing a value of an arrow function

Question

I am trying to figure out a typescript situation where I am trying to define the output of a arrow function.

let isThisTrue:boolean = () => {
  return true;
};

console.log(isThisTrue());

Can I only define a type when I am passing a value to a method? I know this example isn't really practical but I am just trying to grasp why I am unable to say that the expected output is a boolean.

Fiddle: https://jsfiddle.net/mn9efLtg/

Error: Uncaught SyntaxError: Unexpected token :

Ori Drori · Accepted Answer · 2018-09-20 04:45:40Z

3

The type of isThisTrue is function. The result value of the function is Boolean (typescript playground):

const isThisTrue = ():boolean => {
  return true;
};

console.log(isThisTrue());

answered Sep 20, 2018 at 4:45

Ori Drori

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2 Comments

SBB Over a year ago

Awesome, that makes more sense. I assume const is also used because a function should always be the same in this case.

T.J. Crowder Over a year ago

@SBB - That's entirely up to you. If you intend to change isThisTrue at some point later in the code, use let. If you don't, then you can use const to A) document that fact for future maintainers of the code, and B) have the JavaScript engine enforce it (by raising an error) if you accidentally do try to change its value. :-)

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