this - Javascript lost context when assigned to other variable

Question

Why in javascript if you reference objects method to some variable it loses that objects context. Can't find any link with explanation what happens under the hood. Except this one which states: ‘this’ refers to the object which ‘owns’ the method which doesn't seam to be true.

var Class = function() {
    this.property = 1
}

Class.prototype.method = function() {
    return this.property;
}

var obj = new Class();
console.log(obj.method() === 1);

var refToMethod = obj.method; // why refToMethod 'this' is window


console.log(refToMethod() !== 1) // why this is true?

var property = 1;
console.log(refToMethod() === 1)

Answer 1

It depends on how a function is called. If a function is not referenced through being an attribute of an object (e.g. refToMethod) then it will be assigned the "Global context" which is window. However, when a function is an attribute of object (e.g. obj.method), we refer to it as a method, and it is implicitly assigned the context of it's parent object.

JavaScript's context is unlike many languages in that you can override it easily using either .call() or .apply(). Furthermore, ECMAScript 5 introduced a new .bind() method to allow you to create copies of methods which are always bound to the same context. See MDN for more.

var obj = new Class();
obj.method(); // 1;

var unbound = obj.method;
unbound(); // undefined;

// Call and Apply setting the context to obj.
unbound.apply(obj); // 1
unbound.call(obj); // 1;

// ECMAScript 5's bind
var bound = unbound.bind(obj);
bound(); // 1;