为什么在 JavaScript 中的 window object 中添加了全局变量？

Question

Why are global variables added to a window object in JavaScript?

var a = 1;
console.log(window.a);

ECMAScript 2015 language specification does not say that declared var or function is added to window global objects. However, I would like to know why the global variable declared var is added as a property to the window object.

What I'm curious about is that it has nothing to do with ECMAScript language specification?

I used a translator, so please understand if it's weird.

Answer 1

It actually is noted in the specification here , when a variable is assigned to when no such identifier exists:

If IsUnresolvableReference(V) is true, then
  a. If V.[[Strict]] is true, throw a ReferenceError 
exception.
  b. Let globalObj be GetGlobalObject().
  c. Return ? Set(globalObj, V.[[ReferencedName]], W, false).

And here , in CreateGlobalVarBinding, which is called when a variable is declared at the top level of a script .

6. Let varDeclarations be the VarScopedDeclarations of script.
...
18. For each String vn of declaredVarNames, do
  a. Perform ? env.CreateGlobalVarBinding(vn, false).

The global object may be the window in a browser, or self in a web worker, or the Node global , etc.

Answer 2

I think the reason why var causes properties to be added to the global object (in the example you provided) has to do with how environments are linked in Javascript; they are linked by outer references , yet the global environment has no outer reference. Perhaps the choice to bind them to the global object, as properties, stems from this lack of outer reference in the global environment.

The global scope is the “outermost” scope – it has no outer scope. Its environment is the global environment. Every environment is connected with the global environment via a chain of environments that are linked by outer references. The outer reference of the global environment is null .

Source: https://2ality.com/2019/07/global-scope.html#the-global-object