JavaScript this referring to global object in strict mode

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I was learning this in JavaScript and according to my understanding inside strict mode this should be undefined but when I try this

"use strict";


function demo () {
    // Here this is Undefined in strict mode
    console.log(this);
}

//Here this refers to the Global Object
console.log(this);

demo();

As mentioned in the comment, console still prints the Window Object except when I call the demo() object. Why is that so?

this javascript
1 Answers

ECMA2015 (ES6) specified that global environment records supply a this value:

8.1.1.4.8 HasThisBinding () Global Environment Records always provide a this binding whose value is the associated global object.

By ECMA2018,this was modified slightly - from table 17 in section 8.1.1.4 Global Environment Records of this edition:

Table 17: Additional Fields of Global Environment Records
...
Field Name: [[GlobalThisValue]]
Value: Object
Meaning: Object The value returned by this in global scope. Hosts may provide any ECMAScript Object Value.

However typical host environments still set the global this value to the global object and I assume will continue to do so.

While strict mode does provide that the this value in an function called simply and directly to be undefined, it does not suppress the creation of a binding for this in the global environment record which will be seen as the this value in global, file-level code.

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