JavaScript Variable declaration predecessor

Variable declaration predecessor:

The so-called variable declaration predecessor is in a scope block, all the variables are placed in the beginning of the block declaration, the following example you can understand

1 var a = 1; 2 function Main () {3     Console.log (a);   14}5 main (); // Output 1

The above code outputs the value of the outer variable A.

1 var a = 1; 2 function Main () {3    Console.log (a); 4     var a = 2; 5 }6 Main ()// output undefined

What do you want to do to output undefined? This is because the script automatically declares the variable declaration before it executes, parsing it as follows:

1 var a = 1; 2 function Main () {3     var A; 4     Console.log (a); 5     A = 2; 6 }

So the output is undefined.

Summary:

For the above problem, we should try to put the variable declaration at the beginning of the scope when we write the JavaScript script, so we can avoid the above problem.

Sometimes we encounter some puzzling problems in the development, in fact, some of us do not understand the implementation principle of JavaScript caused, only to understand, we can reduce such errors, hope this article can help you!

JavaScript Variable declaration predecessor