Variable declaration in JavaScript precedes assignment analysis_javascript tips

As follows

Copy code The code is as follows:

var a = 3;

There are actually two steps:
1 Initialize a to undefined
2 Assign a value 3

Therefore, there will be some "unbelievable" phenomena, that is, variables in JS can be used first and then declared. This is not allowed in Java.

Copy code The code is as follows:

System.out.println(a);
int a = 1;

Compilation fails. But JS can, as follows

Copy code The code is as follows:

alert(a);
var a ;

Although it is undefined, no error will be reported. It shows that a is indeed declared and is undefined.

If it is just "alert(a)" without "var a", the JS engine will report an error.

Copy code The code is as follows:

alert(a);

FF as follows

Although you can use it first and then declare it, this will cause the effect of the assignment to be lost. As follows

Copy the code The code is as follows:

alert(a);
var a = 1;

This time the output is still undefined instead of 1.

Another example,

Copy code The code is as follows:

alert(' a' in window); // true
var a;

Although the code is formally written after alert, the engine still automatically processes the var declaration first. The final output is true.

After understanding this, it is not difficult to understand the results of the following code

Copy the code The code is as follows:

if (!("a" in window)) {
var a = 1;
}
alert(a);