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Javascript local variables and global variables, javascript global variables
In js, the function execution process is not only a local variable that overwrites the global variable, but also related to the declaration in the function. For example:
<Script> var a = 1; function test () {alert (a); // a is undefined! This a is not a global variable. This is because a local variable with the same name has been declared in function scope (the last 4th rows of the function body). // Therefore, global variable a is overwritten, this shows that Javascript performs a complete analysis on the definition of the entire script file before execution. Therefore, before the test () function is executed, // variable a in the function body is directed to the internal local variable. instead of pointing to external global variables. however, at this time, a only declares and has not been assigned a value, so undefined is output. A = 4 alert (a); // a is 4. Is there no suspense? Here, a is a local variable! Var a; // The local variable a declares alert (a) in this line; // a is still 4, because 4 has been assigned to a} test (); alert (a); // a is 1, which is not in function scope. The value of a is the value of the global variable. </script>
First, you can directly run the script in <input type = "text" id = "dd" value = ""> program load.
Var a = document. getElementById ("dd"). value ;;
Id = "dd" cannot be found. If you place the script in <input……> To obtain the value.
If you want to define a as a global variable, the original place will not change in // call the global variable, (2) Add
A = document. getElementById ("dd"). value
You can.
The modified code is as follows:
<Html>
<Head>
<Script type = "text/javascript">
Var a = 1;
Alert ();
Function DD (){
A = document. getElementById ("dd"). value;
Alert ();
A = 2;
Alert ();
}
</Script>
</Head>
<Body>
<Input type = "text" id = "dd" value = "">
<Input type = "button" value = "display" onclick = "DD ()">
</Body>
</Html>
In short, the problem you encountered here is the sequence of program running, rather than the global definition domain.
The running sequence of the Code is
Var a = 1;
Alert ();
Click <input type = "button" value = "show" onclick = "DD ()">
Run the code in DD ().
How to assign the value of a local variable to a global variable in javascript?
Var a = 0; // global
Function (){
Var B = 5; // local
A = B; // value assignment
}
Alert (a); // a is 5 at this time
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