Variable climb in IIFE (lazy parsing)

Question

I get very strange output in the following scenarios:

function test(){ var test=123; console.log(test) } // this output : 123 (function test(){ var test=123; console.log(test) })() // this output: 123 

But using the code below

 (function test(){ test=123; console.log(test) })() //output: function test(){ test=123; console.log(test) } 

Can someone explain.

Answer 1

What you see is not connected with the rise.

Your first example is pretty simple:

 (function test(){ var test=123; console.log(test) })() 

You create a variable (via var ) called test within the scope of the function and assign a value to it, and then output that value.

In your second example var :

 (function test() { test = 123; console.log(test); })(); 

... and therefore test has a completely different meaning: inside a function created by a named function expression, the function name is the identifier that resolves the function. Thus, test in this code is the identifier for the function.

This identifier is read-only when you use a function expression, so your line is test = 123; is ignored, and the console.log line displays a representation of the function (as if test = 123; there was none at all).

I forgot that the identifier is read-only (when creating an expression), but this is: From the specification :

FunctionExpression: functionBindingIdentifier (FormalParameters) {FunctionBody}

• If the functional code for FunctionExpression is strict mode code, let strict be true. Otherwise, let strict be false.
• Let scope be the executable execution context of LexicalEnvironment.
• Let funcEnv be NewDeclarativeEnvironment (scope).
• Let envRec be funcEnv EnvironmentRecord.
• Let the name be StringValue BindingIdentifier.
• Run envRec.CreateImmutableBinding (name, false).
• ...

Note. Step 6: The binding creating the identifier is immutable (cannot be changed).

Note that this is not true for an identifier (binding) created by a function declaration that is mutable; but function expressions and function declarations handle identifiers created by a function name in completely different ways. (For example: A function declaration puts the name in scope, but the function expression does not matter.)