JS variable Promotion Deep Understanding _ basics

The function definition of JavaScript has a feature that scans the statement of the entire function body and "promotes" all declared variables to the top of the function:

' Use strict ';

function foo () {
  var x = ' Hello, ' + y;
  alert (x);
  var y = ' Bob ';
}
Foo ();

Although it is strict mode, the statement var x = ' Hello, ' + y is not an error, because the variable y is stated later. But alert shows hello, undefined, indicating that the value of variable y is undefined. This is precisely because the JavaScript engine automatically promotes the declaration of the variable y, but does not raise the assignment of the variable Y.

For the above foo () function, the JavaScript engine sees code that is equivalent to:

function foo () {
  var y;//elevation variable y declaration
  var x = ' Hello, ' + y;
  alert (x);
  y = ' Bob ';
}

Because of this weird "feature" of JavaScript, when we define variables inside a function, we strictly observe the rule "declare all variables first within a function". The most common practice is to use all the variables used internally by a VAR declaration function:

function foo () {
  var
    x = 1,//x initialized to 1
    y = x + 1,//y initialized to 2
    z, I;//z and I for undefined
  //Other statement:
  F or (i=0 i<100; i++) {
    ...
  }}
}

The above JS variable to promote in-depth understanding of the small series to share all the content, hope to give you a reference, but also hope that we support the cloud-dwelling community.