Complex variable declaration and initialization of variables in Typescript

Question

What does

 private _someVar: (value: any) => void = () => {};

mean?

I think it's "a variable with type as a function that can take any value and return type void" , but what's the deal with = () => {};?

Is it the declaration of variable, like assigning an empty function to it?

If so, why would someone do that?

Can you please provide me some real life example to do so?

Also, it would be really helpful to have few more such examples to make me more comfortable with such declarations.

() => {} is a function that doesn't do anything. It's the initial value of the variable. — JB Nizet
– JB Nizet, Commented Jul 9, 2018 at 18:25
A use case: a property that specifies an event handler. By default, nothing happens when the event occurs; but it could if the consumer of the class sets one. — Heretic Monkey
– Heretic Monkey, Commented Jul 9, 2018 at 18:30
You guessed what it does correctly. I can't answer why they need an empty function. — Peter Olson
– Peter Olson, Commented Jul 9, 2018 at 19:20

George Helyar · Accepted Answer · 2018-07-09 18:26:30Z

1

You are correct. It's a function that takes an any and returns void.

The = () => {} is a default value, assigning an empty function to it, so that if it is not overwritten, it can still be called but it won't do anything.

answered Jul 9, 2018 at 18:26

George Helyar

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2 Comments

User985614 Over a year ago

Thanks, but what could be the place we need such function ?

George Helyar Over a year ago

You could use it to set a callback, for example. The usage does seem a bit weird. The only reason I can think of to do it this way is so that you don't have to check if the function is set before calling it. For insight into why the author implemented it this way, you would have to ask the author.

Noura Almousa · Accepted Answer · 2018-07-09 18:48:56Z

0

This is an arrow function. Arrow functions are a short syntax, introduced by ECMAscript 6, that can be used similarly to the way you would use function expressions. In other words, you can often use them in place of expressions like function (parameters1 , parameter 2,....) {syntax......} .but in arrow function you can write (parameter1, parameter2 ,...) => (syntax)

answered Jul 9, 2018 at 18:48

Noura Almousa

1

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