JavaScript Quirks Analysis

It's interesting to learn about some of the programming quirks of JavaScript, and it's worth summarizing:

1. Unknown variable name Create global variable

in our usual programming of JavaScript, some people are not very formal, when defining variables, the direct definition of "variable name = value" is not preceded by "Var", JavaScript automatically defines it as a global variable view

But in ECMAScript5 's rigorous mode get a warning:

For example: function f () {' Use strict '; foo = 123} f () Referenceerror:foo are not defined

2. Two null values of underfined and NULL

For "null" or "null reference", most programming languages have only one value. For example, in Java null , 2 values (undefined and null) are given in JavaScript

underfined:

<script type= "Text/javascript" >        var  s;        Console.log (s);     </script>

Results:

Similarly: If there is no value in the operation of the value parameter, a undefined is automatically assigned.

  <script type= "Text/javascript" >       functionreturn  x}       Console.log (x);   </script>

Running results in Firefox:

Running results in IE: Khan! Firefox at least to a undefined,ie directly on the judgment is undefined, this mechanism is too lazy ...

null: Is used by the developer to explicitly indicate that a value is missing

During the development process, we may need to pass this parameter without a value, and set the parameter equal to NULL for this scenario.

School check: Does a variable have a value?

 <script type= "Text/javascript" > //  var   X;  if   (x) {Console.log ( ' has value ' 
    
     ); 
     else   {Console.log ( ' no value ' 
   
    ); }  </script> 
   

If you want to know the variable x   undefined   null . Fortunately, the two values are false. Therefore, it is possible to test whether the two items are true at the same time using only one judgement,   false ,   -0 ,   +0 ,   NaN   and "as null values. If this is not what you want, then you cannot use the school check method above. There are 2 ways to choose

Some people like to use "! =" for parameter checking

JavaScript Quirks Analysis