[Javascript] this in Function Calls

In most cases, the value of a function's this argument is determined by how the function is called. This lesson explains what thisrefers to when we call plain function. Marius points out how functions behave differently in strict and non-strict mode. "use strict"mode defaults this to undefined and prevents us from assigning values to undefined. We must call functions as a constructor to assign their this value correctly.

"use strict";

console.log(this === global)  // false, in REPL this === global
console.log(this === module.exports) // true

function Person(firstName, lastName) {
    console.log(this === global) // without 'use strict', true; with strict mode, false
    console.log(this === undefined) //without 'use strict', false; with strict mode, true
}

Person()

Inside a function, 

• strict mode, 'this' is undefined
• without strict mode, 'this' is global

"use strict";

console.log(this === global)  // false, in REPL this === global
console.log(this === module.exports) // true

function Person(firstName, lastName) {
    console.log(this === global) // without 'use strict', true; with strict mode, false
    console.log(this === undefined) //without 'use strict', false; with strict mode, true
    this.firstName = firstName;
    this.lastName = lastName;
}

const person = new Person("Jane", "Doe");
console.log(person);
console.log(global.firstName); //undefined
console.log(global.lastName); //undefined