Cannot read property X of undefined on call

Question

So I have a class

class A {
    test() {
        return 1;
    }
    foo() {
        return this.baz(this.bar)
    }
    bar() {
        return this.baz(this.test);
    }
    baz(f){
        return f(); 
    }
}

And when I call the method foo

var a = new A();
a.foo();

I get

Uncaught TypeError: Cannot read property 'baz' of undefined
    at bar (<anonymous>:9:15)
    at A.baz (<anonymous>:12:10)
    at A.foo (<anonymous>:6:15)
    at <anonymous>:1:3

How appears that this becomes undefined after method f() is being called, and how can I fix this?

Ultimately it becomes something like: `var bar = a.bar; bar();` So, `this` will be undefined. Example 3 from the accepted answer: [How does the “this” keyword work?](https://stackoverflow.com/questions/3127429) — adiga, Oct 24 '19 at 09:21

score 1 · Answer 1 · answered Oct 24 '19 at 09:24

By doing that:

return this.baz(this.bar)

you are just passing a function without specifying what this its this refers to (without its context). So ultimately, when you enter the baz(f) method, you get a function as a parameter, not method of the class A. So its this is undefined. To fix it, you have two options, first is to change the foo() method:

foo() {
    return this.baz(this.bar.bind(this))
}

and a second is to change the baz(f) method:

baz(f){
    return f.call(this); 
}

Both solutions will work in this case. Hope it helps

score 0 · Answer 2 · answered Oct 24 '19 at 09:23

0

It was never defined. You call function without context - so does not have "this"

this.apply(f) would bind it to object

answered Oct 24 '19 at 09:23

Konstantin Pribluda

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score 0 · Answer 3 · answered Oct 24 '19 at 09:26

You can also use arrow function to solve this problem.

class A {
    test =()=> {
        return 1;
    }
    foo=()=> {
        return this.baz(this.bar)
    }
    bar=()=> {
        return this.baz(this.test);
    }
    baz=(f)=>{
        return f(); 
    }
}

const a = new A()
console.log(a.foo())

Cannot read property X of undefined on call

3 Answers3