执行上下文JavaScript的可变环境

Question

In the sec. 10.3 describes the components of execution context as the follwoing:

The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments. When an execution context is created its LexicalEnvironment and VariableEnvironment components initially have the same value. The value of the VariableEnvironment component never changes while the value of the LexicalEnvironment component may change during execution of code within an execution context.

Ok, let we have code snippet:

{
    alert(o.prop);
    var o={prop: 'prop'};
}

My understanding of this code snippet:

When control is transferred to this code snippet a corresponding execution context will be created and pushed to stack. This context is become a running execution context (called cont ). During the cont is creating the VariableEnvironment of cont is creating. After the cont 's creation this code starting to execute.

But as said in sec.10.3

The value of the VariableEnvironment component never changes

Thus we can assume that the VariableEnvironment never changed after creation of cont . Ie environment record of cont 's VariableEnvironment contains all binding created by VariableStatement and FunctionDeclarartion initially. Hence we can apply to object by reference o . But I have TypeError: o is undefined .

Question: Why TypeError , described above is caused? I'm expected that prop will be displayed with alert message because the value of the VariableEnvironment component never changes hence environment record of the VariableEnvironment never changes, hence all bindings of this record is immutable.

Might I dont understand mean of value of EnvironmentRecord correctly?

Answer 1

First, your code is just definining a block , and JavaScript doesn't have block scope; environment records are associated with global scope and functions, not blocks.

So let's assume that's actually a function so it creates a new scope and environment record:

function foo()
{
    alert(o.prop);
    var o={prop: 'prop'};
}

That's treated exactly as though it were this:

function foo()
{
    var o;
    o = undefined;
    alert(o.prop);
    o={prop: 'prop'};
}

Which should clarify things a bit. :-) You're trying to dereference o as though it referred to an object, but it doesn't (yet); its value is undefined .

This is because all var declarations are processed upon entry to the execution context, and all variables are initialized to undefined , before any step-by-step work is done. Any initializer associated with a var (eg, var o = ... ) is really an assignment, and happens later when that code is reached in the step-by-step execution of the code.

More (on my blog) :

• Poor misunderstood var