What are methods on JavaScript objects, and what does this refer to?

A method is just a function stored as a property of an object, and inside it this refers to the object that owns the method.

When a function is stored as one of an object's properties, it's called a method — it gives the object a behaviour, not just data. Inside a method, the special keyword this refers back to the object the method belongs to (its "owner"), so the method can read that object's other properties:

let person = {
    firstName: 'Hans',
    name: 'Meier',
    fullName: function() {
        return this.firstName + ' ' + this.name;
    }
};

console.log(person.fullName());   // 'Hans Meier'

Here this.firstName and this.name reach into the same person object the method is attached to.

Two things to watch:

• Always call a method with parentheses: person.fullName() runs it and returns 'Hans Meier'. Without the (), person.fullName gives you the function itself, not its result — a frequent mistake.
• Modern JavaScript offers a shorthand that drops the function keyword and the colon:

fullName() {
    return this.firstName + ' ' + this.name;
}

Go deeper:

• MDN: this — why this depends on how a function is called, and how arrow functions inherit it from the enclosing scope.