JavaScript中的执行上下文和对象

Question

There's something about object, execution context in JS that I don't understand.

When we create an object, does it create an execution context ? since an execution context is created when a function is invoked. And if it doesn't, so the object is just like the others variable in the actual execution context ?

Thank you.

Answer 1

When we create an object, does it create an execution context ?

No.

since an execution context is created when a function is invoked.

That's true, but creating an object is different from invoking a function.

And if it doesn't, so the object is just like the others variable in the actual execution context ?

The object exists in memory, and a reference to it exists in any variable or property you stored it in. If you store it in a variable, that variable is held in a lexical environment object associated the execution context where the variable was declared.

A concrete example might help:

function foo() {
    var n = 42;
    var o = {};
    console.log(n, o.toString()); // 42, [object Object]
}
foo();

Calling foo creates an execution context and a lexical environment object associated with it. The n and o variables are bindings stored in that lexical environment. The n binding's value is the primitive number 42. The o binding's value is a reference to the object. The object itself exists elsewhere in memory.

+−−−−−−−−−−−−−−−−−−−−+
| Execution Context  |
+−−−−−−−−−−−−−−−−−−−−+   +−−−−−−−−−−−−−−−−−−−−−+
| Current Lex Env    |−−>| Lexical Environment |
| (some other stuff) |   +−−−−−−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−+   | n: 42               |   +−−−−−−−−+
                         | o                   |−−>| Object |
                         | (some other stuff)  |   +−−−−−−−−+
                         +−−−−−−−−−−−−−−−−−−−−−+

But again, that context and its lexical environment are created by the call to foo , not by creating an object.

Once foo returns, if no closures were created within foo , the execution context and its associated lexical environment are eligible for garbage collection.

Getting a bit far afield of your question: If we'd created a closure within foo and retained a reference to it even after foo returned, the lexical environment would be retained by that closure; more on closures in this question and its answers .