Understanding and flexible operation of "| |" and "&&" in Javascript-javascript

You must have seen this code: a = a| | " XXX ".

It is actually equivalent to the following three types of code:

A = a | | "XXX";

And:

if (!a) {   a = "xxx";}

And:

if (a = = NULL | | a = = "" "| | A = = undefined) {    a = "xxx";}

How to understand three kinds of code. You must first understand the question: what happens to data types in JavaScript when they are converted to bool types?

In JavaScript, data types can be classified as "true" and "false." That is, true when the truth is converted to the bool type; False if the value is converted to type bool. The following values are used for some common data types when converting to type bool:

Data type	Value after conversion to bool
Null	FALSE
Undefined	FALSE
Object	TRUE
function	TRUE
0	FALSE
1	TRUE
Numbers outside of 0, 1	TRUE
String	TRUE
"" (empty string)	FALSE

In an If expression, JavaScript first converts the conditional expression to the bool type, and the expression is true if the logic in the if is executed, otherwise skipped.

Then take a look at the two JavaScript expressions "&&" and "| |". JavaScript is a type prophecy, so the two expressions in JavaScript may not be the same as in other languages.

The && algorithm is as follows:

If the value of the && left expression is true, the value of the right-hand expression is returned, otherwise the value of the left-hand expression is returned.

Other words:

var i = "" && "truth";//-i = ""; i = "truth" && "other Truth";//-i = "Other true value" I = "truth" && ""; -i = ""

|| the algorithm is as follows:

if | | The value of the left-hand expression is true, and the value of the right-hand expression is returned otherwise.

var i = "" | | " True value ";//-I =" truth "; i =" Truth "| | "Other truth";//-I = "truth" i = "truth" | | ""; -i = "truth"

So you can understand A = a | | "XXX"; Logic: If A is a false value (equal to null, an empty string "" ...). ), then "XXX" is assigned to A; Otherwise, assign the value of a to the a itself.

The following examples apply, using | | and && to streamline the program:

var parameter= "";  function Test (parameter) {      //return truth      return true;  }    Truth Operation  function operate1 (parameter) {      return "truth Operation";  }    False value operation  function operate2 (parameter) {      return "false value operation";  }    var result=test (parameter) &&operate1 (parameter);  Result=test (parameter) | | Operate2 (parameter);    Equivalent to  result=test (parameter) operate1 (parameter): operate2 (parameter);    alert (result); The truth Operation    //is also equivalent to  if (test (parameter)) {      result=operate1 (parameter);  } else{      result=operate2 (parameter);  }    Alert (Result)//truth operation  

JACK

@@ 12/11/2014 NJ USA

　　

Understanding and flexible operation of "| |" and "&&" in Javascript-javascript