What does this refer to in a JavaScript function?

Question

function Box(width, height)
{
  this.width = width;
  this.height = height;
}

var myBox = new Box(5,5);

1. What is the new keyword doing here technically? Is it creating a new function? Or is it creating a new object and applying the function to it?

2. If so then this is a way to create a "Box", does this mean the this keyword is actually referring to the object myBox?

Answer 1

It's creating a new object, using Box as its constructor. The value of this in this case (when the function is called with the new keyword) is the new instance being constructed. This new object will inherit from whatever is defined as Box.prototype (the default being Object.prototype).

I said in this case, because in JavaScript the value of this is determined by how the function is called. I recommend reading the MDN page on this for more information.

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