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Explain the value of "this"

importance: 3

In the code below we intend to call obj.go() method 4 times in a row.

But calls (1) and (2) works differently from (3) and (4). Why?

let obj, method;

obj = {
  go: function() { alert(this); }

obj.go();               // (1) [object Object]

(obj.go)();             // (2) [object Object]

(method = obj.go)();    // (3) undefined

(obj.go || obj.stop)(); // (4) undefined

Here’s the explanations.

  1. That’s a regular object method call.

  2. The same, parentheses do not change the order of operations here, the dot is first anyway.

  3. Here we have a more complex call (expression).method(). The call works as if it were split into two lines:

    f = obj.go; // calculate the expression
    f();        // call what we have

    Here f() is executed as a function, without this.

  4. The similar thing as (3), to the left of the dot . we have an expression.

To explain the behavior of (3) and (4) we need to recall that property accessors (dot or square brackets) return a value of the Reference Type.

Any operation on it except a method call (like assignment = or ||) turns it into an ordinary value, which does not carry the information allowing to set this.