JavaScript is very flexible with regards to checking for "null" values. I'm guessing you're actually looking for empty strings, in which case this simpler code will work:
if(!pass || !cpass || !email || !cemail || !user){
Which will check for empty strings (""), null, undefined, false and the numbers 0 and NaN.
Please note that if you are specifically checking for numbers, it is a common mistake to miss 0 with this method, and num !== 0 is preferred (or num !== -1 or ~num (hacky code that also checks against -1)) for functions that return -1, e.g. indexOf).
JavaScript is very flexible with regards to checking for "null" values. I'm guessing you're actually looking for empty strings, in which case this simpler code will work:
if(!pass || !cpass || !email || !cemail || !user){
Which will check for empty strings (""), null, undefined, false and the numbers 0 and NaN.
Please note that if you are specifically checking for numbers, it is a common mistake to miss 0 with this method, and num !== 0 is preferred (or num !== -1 or ~num (hacky code that also checks against -1)) for functions that return -1, e.g. indexOf).
To check for null SPECIFICALLY you would use this:
if (variable === null)
This test will ONLY pass for null and will not pass for "", undefined, false, 0, or NaN.
Additionally, I've provided absolute checks for each "false-like" value (one that would return true for !variable).
Note, for some of the absolute checks, you will need to implement use of the absolutely equals: === and typeof.
I've created a JSFiddle here to show all of the individual tests working
Here is the output of each check:
Null Test:
if (variable === null)
- variable = ""; (false) typeof variable = string
- variable = null; (true) typeof variable = object
- variable = undefined; (false) typeof variable = undefined
- variable = false; (false) typeof variable = boolean
- variable = 0; (false) typeof variable = number
- variable = NaN; (false) typeof variable = number
Empty String Test:
if (variable === '')
- variable = ''; (true) typeof variable = string
- variable = null; (false) typeof variable = object
- variable = undefined; (false) typeof variable = undefined
- variable = false; (false) typeof variable = boolean
- variable = 0; (false) typeof variable = number
- variable = NaN; (false) typeof variable = number
Undefined Test:
if (typeof variable == "undefined")
-- or --
if (variable === undefined)
- variable = ''; (false) typeof variable = string
- variable = null; (false) typeof variable = object
- variable = undefined; (true) typeof variable = undefined
- variable = false; (false) typeof variable = boolean
- variable = 0; (false) typeof variable = number
- variable = NaN; (false) typeof variable = number
False Test:
if (variable === false)
- variable = ''; (false) typeof variable = string
- variable = null; (false) typeof variable = object
- variable = undefined; (false) typeof variable = undefined
- variable = false; (true) typeof variable = boolean
- variable = 0; (false) typeof variable = number
- variable = NaN; (false) typeof variable = number
Zero Test:
if (variable === 0)
- variable = ''; (false) typeof variable = string
- variable = null; (false) typeof variable = object
- variable = undefined; (false) typeof variable = undefined
- variable = false; (false) typeof variable = boolean
- variable = 0; (true) typeof variable = number
- variable = NaN; (false) typeof variable = number
NaN Test:
if (typeof variable == 'number' && !parseFloat(variable) && variable !== 0)
-- or --
if (isNaN(variable))
- variable = ''; (false) typeof variable = string
- variable = null; (false) typeof variable = object
- variable = undefined; (false) typeof variable = undefined
- variable = false; (false) typeof variable = boolean
- variable = 0; (false) typeof variable = number
- variable = NaN; (true) typeof variable = number
As you can see, it's a little more difficult to test against NaN;
Basic JS question: when to check for undefined, null, etc
TL;DR: Use value != null. It checks for both null and undefined in one step.
In my mind, there are different levels of checking whether something exists:
0) 'property' in object - Returns true if the property exists at all, even if it's undefined or null.
-
object.property !== undefined- Returns true if the property exists and is not undefined. Null values still pass. -
object.property != null- Return true if the property exists and is not undefined or null. Empty strings and 0's still pass. -
!!object.property- Returns true if the property exists and is "truthy", so even 0 and empty strings are considered false.
From my experience, level 2 is usually the sweet spot. Oftentimes, things like empty strings or 0 will be valid values, so level 3 is too strict. On the other hand, levels 0 and 1 are usually too loose (you don't want nulls or undefineds in your program). Notice that level 1 uses strict equality (!==), while level 2 uses loose equality (!=).
More on reddit.comUndefined vs null
[AskJS] Why hasn't the null type bug been fixed?
Javascript bind function first parameter null vs this
Is null false in JavaScript?
What is a null check?
What is a strict null check?
Videos
So I'm usually more of a server side developer, but lately I've been working with more of the client code at work. I understand what undefined and null are in JavaScript, but I find myself always checking for both of them. In fact, when checking if a String property exists, I end up writing this:
if(value !== undefined && value !== null && value !== '')
I figure there is a better way than this, and it's probably because I'm not 100% clear of when to check for what. So if someone could help fill me in here on the rules of when to check for undefined vs null, that would be great.
TL;DR: Use value != null. It checks for both null and undefined in one step.
In my mind, there are different levels of checking whether something exists:
0) 'property' in object - Returns true if the property exists at all, even if it's undefined or null.
-
object.property !== undefined- Returns true if the property exists and is not undefined. Null values still pass. -
object.property != null- Return true if the property exists and is not undefined or null. Empty strings and 0's still pass. -
!!object.property- Returns true if the property exists and is "truthy", so even 0 and empty strings are considered false.
From my experience, level 2 is usually the sweet spot. Oftentimes, things like empty strings or 0 will be valid values, so level 3 is too strict. On the other hand, levels 0 and 1 are usually too loose (you don't want nulls or undefineds in your program). Notice that level 1 uses strict equality (!==), while level 2 uses loose equality (!=).
I would just say
if (value) {
// do stuff
}because
'' || false
// false
null || false
// false
undefined || false
//falseEdit:
Based on this statement
I end up writing this: if(value !== undefined && value !== null && value !== '')
I initially assumed that what OP was really looking for was a better way to ask "is there a value?", but...
if someone could help fill me in here on the rules of when to check for undefined vs null, that would be great.
If you're looking to see if something is "truthy":
if (foo.bar) {
alert(foo.bar)
}This won't alert if value is '', 0, false, null, or undefined
If you want to make sure something is a String so you can use string methods:
if (typeof foo.bar === 'string') {
alert(foo.bar.charAt(0))
}This won't alert unless value is of type 'string'.
So.. "when to check for undefined vs null"? I would just say, whenever you know that you specifically need to check for them. If you know that you want to do something different when a value is null vs when a value is undefined, then you can check for the difference. But if you're just looking for "truthy" then you don't need to.