JS pre-compilation, global variables, local variables related knowledge

1, undeclared variables are assigned, this variable is the global variable of the Window object all

Console.log (a);   A is not defined

a=10

Console.log (a);  10 equivalent to Console.log (WINDOW.A)

function Test () {
var a = b = 123;  
}

Test ();

Console.log (b)  //123  

Console.log (a)   //a is not defined

2. All declared variables are properties of the Window object.

var aaa=111;
var bbb=222;

var ccc=333;

Equivalent to window{aaa:111,bbb:222,ccc:333}

3, pre-compilation occurs in the first moment of function execution, there are the following four steps:

(1), create AO (Activation object/Execution period context) objects;

(2), find formal parameter and variable declaration, the variable and formal parameter name is AO object attribute name, value is undefined;

(3), the integration of the actual parameters and formal parameters;

(4), in the function body found function declaration, the value assigned to the function body;

function fn (a) {
Console.log (a); //? A () {}

Console.log (a);   123
function A () {}
Console.log (a);   123
var B = function () {}
Console.log (b);   //? () { }
}
fn (1);

JS pre-compilation, global variables, local variables related knowledge