this keyword relates to the context on which it is run.
person.sayHello() ⇒ context here is person
person.spouse.sayHello(); ⇒ context is spouse
sayHello() ⇒ context is window for web browsers, globalThis for Node.js >=12 or global for Node.js < 12.

Given this example:

function-context-this.js

function sayHello() {
  console.log(this.name, this.age);
}
 
const person = {
  name: 'Alice',
  age: 30,
  sayHello: sayHello,
  spouse: {
    age: 32,
    sayHello: sayHello
  }
};
 
person.sayHello(); // Alice, 30
person.spouse.sayHello(); // undefined, 32
sayHello(); // undefined, undefined

When you call person.spouse.sayHello(), this.name evaluates to undefined since there's no name property on the spouse object, and this.age evaluates to 32.

In non-strict mode, this refers to the global object, and since there's no name or age property on the global object, both will be undefined.
So sayHello(); in non strict mode will display undefined, undefined. But in strict mode, it will throw an error.

"use strict";
 
function sayHello() {
  console.log(this.name, this.age);
}
 
// ... rest of the code ...
 
sayHello(); // TypeError: Cannot read properties of undefined (reading 'name')