JS face question (i.)

1. JS Six basic data types: string Boolean Number object undefined function

typeof returns a string of six kinds: string Boolean Number object undefined function

var a=1; if (function  f () {}) {    x+ =typeof  F;} // value of x: "1undefined"
At this point function f () {} is not a functional declaration, it is converted to true, and internal f is undefined

2, instanceof

Instance a instanceof B (A is an instance of B), returns True, False

[] instanceof Array;//true

3 . Delete to remove the member variable of the object

(function(x) {delete  x;   Falsealert (x);}) (1)// results: 1
Because delete failed to delete X

4. js comma operator

var a= (All-in-one);//Final A has a value of 3, select the last value

5, JS pre-analysis　

(function  f () {    function aaa () {return 1};     return aaa ();     function aaa () {return 2};}) (); // The returned result is 2 because the code is pre-parsed before execution, and the following function declaration overrides the previous

6. Function parameters equivalent to local variables

var a=10; function aaa (a) {//A is equivalent to a copy of the global variable, which is a different parameter from the global variable A, a local variable     a+=3;} aaa (a); alert (a);// At this point, we find the global a, basic type of copy operation, Global A has never been changed//10

var a=10;
function aaa () {
    a+=3;//assignment operation, JS has no block-level scope, so changes the value of global variable A
}
AAA ();
alert (a);
13

JS face question (i.)