'this' is undefined when passing function signature as parameter

Asked Nov 09 '21 at 17:07

Active Nov 09 '21 at 17:07

Viewed 26 times

In the code bellow I'm passing a method of a class instance as callback argument to a another function. But this is undefined when I pass just the method signature. So I wonder what changes this according to the callback argument format. Is there some JS docs for this specific case?

class MyClass {
    foo = 'foo';
    bar() {
        console.log('This is:');
        console.log(this);
    }
}

const handler = (callback) => {
    callback()
}

const myClass = new MyClass()

handler(myClass.bar)
handler(() => myClass.bar())

Output:

This is:
undefined

This is:
MyClass {foo: 'foo'}

asked Nov 09 '21 at 17:07

Jeff Pal

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2

`this` is only set when you call a function with the syntax `object.function()`. Just passing `object.function` as a value doesn't bind `this`. – Barmar Nov 09 '21 at 17:09
Arrow functions don't have a `this` – Robert Nov 09 '21 at 17:11
@RobertRocha - The `this` in question is within `MyClass`'s `bar` method, which does. – T.J. Crowder Nov 09 '21 at 17:19
@RobertRocha Statements like this makes little to no sense for js newcomers. Does it mean OP can't use `this` inside an arrow function or what? – Yury Tarabanko Nov 09 '21 at 17:24

'this' is undefined when passing function signature as parameter

0 Answers0