Small details about JS variable Declaration

Small details about JS variable Declaration
 

 
If you try to assign a value to an object that has never been declared, it is implicitly declared as a global object. For example:
 

 
(Function (){
 

 
S = 'abc ';
 

 
})();
 

 
Alert (s); // ABC will pop up
 

 
If you try to read an undeclared object, JS will report an error. For example:
 

 
Alert (a); // No dialog box is displayed, and an error is reported.
 

 
This conclusion is written in the rhino book. However, it is interesting that in IE, if you try to read an undeclared object, no error will be reported, and of course no JS execution will continue. In fact, this is really terrible. If you don't execute it, you won't tell you why.
 

 
However, this conclusion is strong, so the following statements are common mistakes:
 

 
If (! A ){
 

 
// Do something ....
 

 
}
 

In this way, sometimes the original intention is: if the variable A does not exist, execute the statements in it. However, if A is not defined, an error will be reported in Firefox; no error will be reported in IE, but no subsequent statements will be executed.
 

 
So how can we detect whether a variable is defined? A clever way is:
 

 
If (! Window. ){
 

 
// Do something ....
 

 
}
 

 
This method is actually amazing, because an error is reported when you read an undeclared variable, but no error is returned when you read an undeclared attribute. Why does tianxiao JS stipulate such a strange syntax.
 

 
However, this method only applies to global variables. If it is a local variable in a function, use typeof to judge it:
 

 
If (typeof A = 'undefined '){
 

 
// Do something ....
 

 
}
 

 
Therefore, the conclusion is that you should not use the variable before it is undefined.
 

 
Next, let's look at another example:
 

VaR A = "I'm out ";
 

 
(Function (){
 

 
Alert ();
 

 
VaR A = "I'm in ";
 

 
})();
 

 
Okay, ask a question. Run this section.Code, What text will pop up?
 

 
Answer 1: "I'm out" is displayed ". Error. JS has no block-level scope. No matter where the VaR statement is, it is defined throughout the function.
 

 
Answer 2: "I'm in" is displayed ". Error. Review the above sentence. It is defined only in the entire function body, but only after being assigned a value.
 

 
The correct answer is "undefined ". Is it dizzy? Try it on your own.
 

 
In short, repeat the previous conclusion: Do not use the variable before it is undefined.
 

 
The last question I cannot understand:
 

 
Write this code in the HTML document:
 

<Script Type= "Text/JavaScript">
 

 
Window ['A'] ='Hi';
 

 
</Script>
 

 
<SCRIPT type ="Text/JavaScript"Src ="Out. js"> </SCRIPT>
 

 
<SCRIPT type ="Text/JavaScript">
 

 
Alert ();
 

 
</Script>
 

 
Then write this sentence in out. JS:
 

 
If (false ){
 

VaR A = 'hello ';
 

 
}
 

 
Run ff and IE6 respectively to see what you get.
 

 
In ff, "Hi" is displayed, but in IE6, "undefined" is displayed ".
 

 
Amazing, right? By syntax, A cannot be undefined in any case. But IE6 will.
 

 
This is not the case if you write both statements in the same file.
 

 
This is not the case if you change window. a In out. js or change the previous one to VaR.
 

 
If you move var A in out. js out of the IF statement or change the if condition to true, this is not the case.
 

 
This bug is very harmful. Therefore, you can only set a good habit for yourself, that is, do not declare variables in conditional statements. Although doing so, it is not prohibited in syntax.
 

From: Wheat study room