||的正确用法是什么？ 在JavaScript中

Question

In the code below, because s is null d = "test" but if s = "hello" then d would = "hello".

Is this correct as it works? what is the correct way to use ||

var s = null;

var d = s || "test";

alert(d);

Answer 1

|| is "or" ; and understanding what happens here is a bit trickery

var a=false;
var b=true;
result=a||b

will give "result" true (as b is true). What happens is:

• 'a' is checked - it is false
• 'b' is checked AFTERWARDS (as no "true" result has been obtained yet, and ONE "true" result would suffice to make the whole || operator be true) - the value of it will be assigned to the left side

if you had

   var a=true;
   var b="test";
   result=a||b

result will yield true; as no other value needs to be checked by the logic of "||"

with

var a=null;
var b="test";
result=a||b;

a will be checked first - it is null, which converts to "false". b is "test", which is non-null, and converts to "true". so the value of b will be assigned.

And yes, this is a correct way to use || ; the feature you use is also called short-circuit evaluation (as it stops evaluating the boolean expression as early as possible)

Answer 2

This works, but if s evaluates to a 'falsy' value, you'll get your default, which might not be what you intended. A more robust, but wordy idiom is

d = (typeof s === "undefined") ? "test" : s;

Answer 3

Yes it is correct unless s is allowed to be blank or 0 which are also falsy values

var s = null;
var d = s || "test";

var s = 0;
var d = s || "test";

var s = "";
var d = s || "test";

All will result in d being "test"

Answer 4

|| is a logical operator. When s is not null then the condition of (s) is true so d is assigned the value of s, otherwise it is assigned 'test'

Answer 5

|| is the OR operator in javascript
so a||b means a OR b in simple terms
explanation of question you have asked is that id you simply do somethings like these in js you will ultimately get in the else block

if(null)
if(undefined)

so s||"test" will mean which ever is not null or undefined which in this case is test

Answer 6

yes correct, || symbols just does the job of OR. when the first condition is true its gonna return that one.. else it will move to the next... simple as it is...