Common pen Test in JS scope, running result problem

Written questions often have running results, and most of the body size is around the scope of expansion, the following summarizes several related questions:

1. The
2. outer variable function can be found inside the variable inside the function (local variable) outside the outer layer is not found.

    function   aaa () { var  a = 10;} alert (a);  // error A is not defined because a is a local variable defined inside the function, the outer (global environment) cannot access the local variable according to the scope relationship.    This will result in an error  

    var  a=10 function   AAA () {alert (a);  function   BBB () { var  a=20;    AAA (); } BBB ();  // 10 because AAA () is executed at the time of the AAA's scope chain a=10;  

    

4. when Var does not work overtime, global variables are automatically generated (not recommended, and the VAR keyword is recommended when defining variables generally)

function aaa () {    var a = B = ten;   B is not defined as a global variable with VAR, so it can be accessed to }aaa () outside the function; // alert (a);//error, A is not definedalert (b); // The equivalent of a     global variable can be accessed // at this time the above code is equivalent to the following code varfunction  aaa () {     = ten;      var a = b;} aaa (); alert (a);//error alert (b);//10

3. Variable search is the nearest principle, find the nearest var definition, the nearest can not find the words in the outer look.

var a=10;     function aaa () {        alert (a);         var a=20;    }    AAA (); // undefined   Nearest principle find var definition, pre-parsing process

4. The precedence is equal when the parameter and local variable have the same name.

var a=10; function aaa (a) {      alert (a);        var a=20;} AAA (a); //   because the parameters and local variables have the same precedence, the local variable 10 is found and then the local variable defined by Var

var a=10;     function aaa (a) {        var a=20;        alert (a);    }    AAA (a); // the  local variable overrides the parameter, so a value is the value of the local variable

var a=10;     function aaa (a) {        a+=3;  The base type/ parameter is equivalent to a local variable, but only partially modifies the value of a, while global access remains the global value     }    aaa (a);    alert (a); //

var a=[1,2,3];
var b=[1,2,3];     function aaa (A, b) {        A.push (4);
b=[1,2,3,4];    }    AAA (A, b);    alert (a); // [1,2,3,4]  reference type is the address of the reference, so local modification affects global values
    alert (b);//[1,2,3]

Common pen Test in JS scope, running result problem