this keyword and getters/setters

[vikramlc]

The this keyword can lead to some headaches in JavaScript - this summary hopefully acts as a remedy.

this refers to different things, depending on where it's used and how (if used in a function) a function is called.

Generally, this refers to the "thing" which called a function (if used inside of a function). That can be the global context, an object, or some bound data/ object (e.g. when the browser binds this to the button that triggered a click event).

1) this in Global Context (i.e. outside of any function)

function something() { ... }
console.log(this); // logs global object (window in browser) - ALWAYS (also in strict mode)!

2) this in a Function (non-Arrow) - Called in the global context

function something() { 
    console.log(this);
}
something(); // logs global object (window in browser) in non-strict mode, undefined in strict mode

3) this in an Arrow-Function - Called in the global context

const something = () => { 
    console.log(this);
}
something(); // logs global object (window in browser) - ALWAYS (also in strict mode)!

4) this in a Method (non-Arrow) - Called on an object

const person = { 
    name: 'Max',
    greet: function() { // or use method shorthand: greet() { ... }
        console.log(this.name);
    }
};
person.greet(); // logs 'Max', "this" refers to the person object

5) this in a Method (Arrow Function) - Called on an object

const person = { 
    name: 'Max',
    greet: () => {
        console.log(this.name);
    }
};
person.greet(); // logs nothing (or some global name on window object), "this" refers to global (window) object, even in strict mode

this can refer to unexpected things if you call it on some other object, e.g.:

const person = { 
    name: 'Max',
    greet() {
        console.log(this.name);
    }
};
const anotherPerson = { name: 'Manuel' }; // does NOT have a built-in greet method!

anotherPerson.sayHi = person.greet; // greet is NOT called here, it's just assigned to a new property/ method on the "anotherPerson" object

anotherPerson.sayHi(); // logs 'Manuel' because method is called on "anotherPerson" object => "this" refers to the "thing" which called it If in doubt, a console.log(this); can always help you find out what this is referring to at the moment!

[vikramlc]

Getters:

const obj = {
  log: ['example','test'],
  get latest() {
    if (this.log.length === 0) return undefined;
    return this.log[this.log.length - 1];
  }
}
console.log(obj.latest); // "test"

Setters:

const language = {
  set current(name) {
    this.log.push(name);
  },
  log: []
}

language.current = 'EN';
console.log(language.log); // ['EN']

language.current = 'FA';
console.log(language.log); // ['EN', 'FA']