Checking if undefined and null in jQuery

问题

I'm a bit confused when checking if null or undefined and if I should be using !== or != and "undefined" or undefined.

Here is some code I'm working on. Where am I going wrong with my null/unudefined etc?

var c = (jQuery(this).prop("target") != null && jQuery(this).prop("target") != undefined && jQuery(this).prop("target").toLowerCase() == "_blank") ? 1 : 0;

Thanks

回答1:

• Both null and undefined are "falsy" values, thus they can be checked like they were boolean values. Thus, there's no sense comparing to null and undefined except for certain situations where you need to know if they are such values.

• when comparing, it's best to use strict comparison (like ===,!== and so on)

• the && in a condition does not evaluate the following condition if the one preceeding it is "falsy".

• You don't even need jQuery since this is your DOM object (presumably an <a>) and you are trying to get the target property:

In the end:

var c = (this.target && this.target.toLowerCase() === "_blank") ? 1 : 0;

回答2:

In general, keep it simple.

To check for undefined, use:

foo === undefined
foo !== undefined

---

To check for null, use:

foo === null
foo !== null

---

To check for either at the same time, use:

foo == null
foo != null

---

And in any case, store your .prop() to a variable to keep it clean. But in your case, if it equals "_blank", then you know it isn't null or undefined, so:

var targ = jQuery(this).prop("target").toLowerCase();

var c = targ === "_blank" ? 1 : 0;

---

Or you could make it even shorter by coercing the boolean to a number:

var targ = jQuery(this).prop("target").toLowerCase();

var c = +(targ === "_blank");

---

These last two solutions are safe because .prop() will always return a string.

回答3:

This is the best way to checking undefined:

if(typeof variable_here != 'undefined'){
   // your code here.
 };

And this is the best way to checking null:

if(variable_here !== null){
       // your code here.
     };

So your code should be like this:

var c = (jQuery(this).prop("target") !== null && typeof jQuery(this).prop("target") !== 'undefined' && jQuery(this).prop("target").toLowerCase() == "_blank") ? 1 : 0;

回答4:

Since undefined would be the variable type, use typeof

var c = (
 $(this).attr("target") != NULL && 
 typeof $(this).attr("target") != "undefined" && 
 $(this).attr("target").toLowerCase() == "_blank"
) ? 1 : 0;

I think, however, that you only need the last check. Is target "_blank", c needs to be 1, otherwise, 0. Does it really matter if target is even set?

Also, use the attr() methode to get attributes, since prop() if for properties like selectedIndex or tagName.

回答5:

Don't get the same property 3 times just to check a value.

var c = 0;
var prop = $(this).prop("target");
if(prop && prop.toLowerCase() === "_blank") c = 1;

回答6:

I got something like this which is not relevent to your question but will help you

var targ = jQuery(this).prop("target").toLowerCase();

now if you want to check whether targ is null or undefined

 var c = (!targ || "") ? 1 : 0

hope this will help you

回答7:

In JavaScript, you are not required to explicitly check if a variable is null or undefined because:

1. null or undefined return false in a boolean expression.

2. JS expressions are evaluated from left to right. So for a || b, if a is false, then only b will be evaluated. But if a is true, b will not be checked. Similarly for a && b if a is false, b will not be evaluated.

Hence, if (a != null) { "do something" } can be written as if (a) { "do something" } or simply a && "do something".

In the same way, it can be used to set a default value when the value is set to null or undefined:

function someFunction(age){
    var age= age|| 18;
}

Reference : https://codereview.stackexchange.com/questions/472/usage-of-the-ternary-operator-with-functions-listening-to-click-events

来源：https://stackoverflow.com/questions/14995904/checking-if-undefined-and-null-in-jquery