在非全局范围内通过“ this”访问声明的变量

Question

var x = 16;
console.log(this["x"]); // 16

I'm ok with this, but:

(function () {
  var y = 16;
  console.log(this["y"]); // undefined
}());

Why we cant access variables via this ?!

I know it's possibe when we assign values, for example:

(function () {
  x = 16; // will assigned as `this["x"] = 16`
  console.log(x); // 16;
}());

What's var problem with non-global scopes?!

Answer 1

You should probably read up on how this works.

Declaring a variable in a local scope using var x = 16 is not the same as doing this.x = 16 . The former example is just a local variable, the latter affects the local context.

Your example:

(function () {
  var y = 16;
  console.log(this["y"]); // undefined
}());

That sets a local variable called y , but then looks for y as defined in the current context, probably window.y . Since the local variable y is not the same as window.y , you get undefined.