This is a spiritual sequel to this article.

Create a class with a method using Traditional function like so. Let’s call this Approach A.

// APPROACH A

class SomeClass {
    constructor() {
        this.someProp = 'someValue';
    }
    someMethod() { // Traditional function
        console.log(this.someProp);
    }
}

Create an instance of that class. When invoking the method on the instance, this refers to the instance. So far, it’s behaving as expected.

let instance = new SomeClass();

instance.someMethod(); // logs 'someValue'

But, as soon as we assign the method to a variable and call that function variable, the method loses its context, and you get Uncaught TypeError: Cannot read property ‘someProp’ of undefined.

let instance = new SomeClass();

let funcVariable = instance.someMethod;
funcVariable(); // logs error

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