JavaScript error-prone knowledge point collation

1. Variable Scope

var a = 1; function Test () {    var a = 2;     // 2 }test ();

The above function scope declares and assigns a, and above the console, so follows the nearest principle output a equals 2.

var a = 1; function test2 () {    //  undefined    var a = 2;} Test2 ();

Although a is declared and assigned in the function scope above, but under the console, a variable is promoted, the output is declared but not yet assigned, so the output is "undefined".

var a = 1; function test3 () {    //  1    = 2;} Test3 ();

A in the upper function scope is re-assigned, is not re-declared, and is under the console, so output a in global scope.

The concept of variable lifting is only applicable to statements that declare variables, and the statement of variable assignment cannot be advanced (if the execution statement can be arbitrarily swapped order, it is destructive imaginable)

We are used to seeing ' var a = 3 ' as a statement. In fact, here are two statements of shorthand, ' var a ' and ' A = 3 ', and actually these two statements are two different types of statements, is done by two different components. The previous sentence is executed during the compile phase, and the latter sentence is executed during the run phase

So, no matter where ' var a ' is written, it will be processed before the code itself is executed. This process is much like a moving process of code, so it is called "variable promotion" by everyone.

2. Type comparison

var arr = [],    = [1//  false

Above two different array comparisons, console is false.

var arr = [],    =//  false

Above two identical array comparisons, console is false.

Reason:

Because two separate arrays are never equal

var arr = [],    = {};console.log (typeoftypeof//  True 

The above uses typeof to compare arrays and objects, because typeof gets null, array, object type is object, so console is true

var arr =instanceof//  trueinstanceof// true

Above uses instanceof to determine whether a variable belongs to an instance of an object, because the array is also one of the objects in JavaScript, so two console is true.

3.this Pointing

My blog

Http://www.cnblogs.com/QianBoy/p/7599202.html

4. Closure issues

var // if there are 5 div on the page  for (var i = 0; i < elem.length; i++) {    function  () {        //  always 5
        };}

Above is a very common closure problem, click on any div pop-up value always 5

var // if there are 5 div on the page  for (var i = 0; i < elem.length; i++) {    (function  (w) {        function c11> () {            ///  0,1,2,3,4        };    }) (i);}

Conclusion

Learning JavaScript is a lengthy process and cannot be accomplished overnight. I hope this article will introduce some of the content can help students learn JavaScript more in-depth understanding and mastering JavaScript grammar, less detours.

JavaScript error-prone knowledge point collation