5

I'm reading a book that says that while it's possible to test whether a variable is defined and has a value by doing:

if(myVar != null && myVar != undefined) {
  ...
}

this is considered “poor style”, and that since both null and undefined are falsey values, we can more easily check if a variable has a value by using the more concise if construct

if(myVar) {
  ...
}

But this confuses me, because while it’s true that null and undefined are falsey values, they are not the only falsey values. In other words, it seems to me that the former if statement says, "run the code as long as the variable isn't null or undefined, whereas the latter if statement is saying, "run the code as long as the variable isn't falsey." Supposing myVar == 0, then the former if statement would run, but the latter if statement would not. Therefore these two if statements are not equivalent. Am I wrong?

EDIT: Here's a screen grab if you want to see their exact wording: testing for null and undefined

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  • 3
    Yes both are not replica of each other. This is all you want? Commented Oct 10, 2015 at 18:20
  • You are right and it is a matter of choice. When you write the code, consider the most common use cases. Commented Oct 10, 2015 at 18:21
  • There is no other way to find out if the variable is null or undefined. Definitely the two snippets that you have shown are not the same. If you myvar variable is a number then you can use isNaN function. Commented Oct 10, 2015 at 18:23
  • @Manwal No, it's not all I want. Given that they are not "replica" (equivalent) of one another, why is the book stating that it's OK to use one in place of the other? Commented Oct 10, 2015 at 18:23
  • 1
    @yroc Are you sure that's exactly what your book says? If so the book is wrong. Commented Oct 10, 2015 at 18:24

2 Answers 2

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5

To check if the variable is either null or undefined, use the not single equal operator checking for null.

if (myVar != null) {

}

However, it's always best practice to use triple equals to be more explicit.

if (!(myVar === null || typeof myVar === 'undefined')) {

}

Comparisons

(undefined == null) => true
(null == null) => true
(undefined === null) => false
(null === null) => true
(0 == null) => false
(0 == undefined) => false
(0 === null) => false
(0 === undefined) => false
(false == null) => false
(false == undefined) => false
(false === null) => false
(false === undefined) => false
(null) => false
(undefined) => false
(0) => false
(false) => false

DEMO: http://jsfiddle.net/xk9L3yuz/3/

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3

There isn't a short way. You first need to check for undefined before null, because 

!myVar

makes the assumption that myVar is previously defined, and will throw 

Uncaught ReferenceError: myVar is not defined

in the event that you never defined that variable.

This check is only required if you're pulling data from some external source and are unsure if the variable exists. If you already defined it somewhere in your code and are for instance waiting for user input to set it, you can take the shortcut of 

!myVar

simply because you know it's already defined because you did it earlier in the code.

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