Variable Instantiation

[Asen Bozhilov]

ECMA-262 3 specification 10.1.3 Variable Instantiation subpoint 3
says:

| If there is already a property of the variable object with
| the name of a declared variable, the value of the property and its
attributes are not changed.
| Semantically, this
| step must follow the creation of the FormalParameterList and
FunctionDeclaration properties.
| In particular, if a
| declared variable has the same name as a declared function or formal
parameter, the variable
| declaration does
| not disturb the existing property.

function execCtx(a)
{
var a = Math.random();
return a;
}
execCtx(10); //random value

If i proper understand specification, returned value from `execCtx'
must be 10 in that case, but in my Firefox 3.5.4 returned value is
Number generated from Math.radom();

That is a bug or feature or maybe i miss something?

Thanks for responses.

 

[Asen Bozhilov]

You missed one tiny sentence in 12.2 (Variable statement)

"A variable with an Initialiser is assigned the value of its
AssignmentExpression when the VariableStatement is executed, not when
the variable is created."

Mmmm thank you so much That explain everything and code like this:

if (!('a' in this))
{
var a = 10;
}
window.alert(a); //undefined

On entering in global execution context `a' will be created.
('a' in this) will be true because property of Global Object with name
`a' been created on enter in execution context.
`a' value will be ever undefined, because VariableStatement never been
executed.

Thanks again.

 

[VK]

if (!('a' in this))
{
    var a = 10;}

window.alert(a); //undefined

On entering in global execution context `a' will be created.
('a' in this) will be true because property of Global Object with name
`a' been created on enter in execution context.
`a' value will be ever undefined, because VariableStatement never been
executed.

This why in a controllable environment I always have the use of
instantiation over assignment as the first section in each block:

var a = '' // future string holder
var b = 0; // future number holder
var c = false; // future boolean holder
var d = null // future object holder

A little bit of a game of course for a loosely typed language but it
helps in many situations plus makes the code more reading friendly.

Just my my two cents...

 

[David Mark]

This why in a controllable environment I always have the use of
instantiation over assignment as the first section in each block:

 var a = ''     // future string holder
 var b = 0;     // future number holder
 var c = false; // future boolean holder
 var d = null   // future object holder

Wouldn't have expected any less from you.

A little bit of a game of course for a loosely typed language but it
helps in many situations plus makes the code more reading friendly.

Just my my two cents...

Worthless at the current exchange rate.