Is there way to create a new global method where I can have variable.myMethod() and it return true or false? Basically, I want to check to see if the variable is undefined and instead of using typeof(variable) == 'undefined' or specifying a function, is it possible to do something like variable.isUndefined() and it would return true or false?
I'm going to go ahead and post this as an answer, so I can go into a bit more detail.
As I mentioned in my comment, you have to be extremely careful about the terms that you use here, as undeclared and undefined mean two very different things.
If a variable is "undeclared", it doesn't exist, thus you cannot attempt to call any methods that might exist on it if it were declared.
If a variable is "undefined", it exists, but it doesn't have a value assigned to it. When this is the case, you can attempt to call methods that may exist on it, however the chances are that they'll fail, since variable doesn't have any value.
Every type in JavaScript is a child of the Object type, therefore you add a method to them, like follows:
Object.prototype.myMethod = function() {
console.log("This is my method");
};
So, in theory, you could create a method to check to see if a value exists, and return a true/false value.
Similarly to what StackOverflow user Barmar pointed out, undefined and null are not children of the Object type, thus your method will not exist.
As other comments have stated, you're probably better of sticking with something like follows:
if (!myVariable) {
// myVariable doesn't have a value
}
I'd like to point out that most of my explanation was unnecessary, as user Barmar pointed out, there is no practical difference between undeclared and undefined.
If a variable is undeclared, it's "value" is essentially read as undefined, thus your method will not exist.
this is a paradox. you can never have a method inside an undefined object because and undefined object does not have anything at all. to make it clear imagine this object
var a = {};
a.b is undefined here, so trying to call a.b.isDefined() can not work because you dont even have b. to make it work you need b defined like this
var a = {b:1,isUndefined:function(){return false}}
so you have to make a generic function that takes objects. This will do the trick
function isUndefined(obj,stringLink){
var arrayLink = stringLink.split(".");
var current = obj[arrayLink[0]];
if(!current)return false;
for(var i =1;i< arrayLink.length;i++){
current = current[arrayLink[i]];
if (!current)return false;
}
return true;
}
it will more or less check for nested object until it reach the target.
if you have
var a = {b:{c:{d:{e:{f:1}}}}}
//this function will do a["b"]["c"]["d"]["e"]["f"]
isUndefined(a,"a.b.c.d.e.f") //gives true
//or you can use
if(a&&a.b&&a.b.c&&a.b.c.d&&a.b.c.d.e&&a.b.c.d.e.f)//lots of work here
I have a slight problem in Javascript with objects being handled in a strange (to say the least) way, unless I'm missing something.
I have an object named state. When I do console.log(state) I get this:
At this point, I try to go through the object using jQuery's each:
$.each(state, function (i, sobj) {
if(mid == sobj.user_id)
//some stuff;
});
However, here I get an error saying TypeError: sobj is null. I found that this is because the trump property within the object (see image above) is initialized to null. With any other value, it works. Obviously I can fix this by initializing it to 0 or something, but this is bugging me on a conceptual level: why shouldn't one of the properties be allowed to have null as its value? It doesn't make sense to me that I should get an error saying the whole object is null when I loop through it like this, just because one of the properties is null. I didn't try to do anything with the property specifically.
So, can someone tell me why this is happening (Google doesn't help) and if there's a way to fix it besides the obvious "don't let anything be null within an object"?
It seems you only want to iterate over the objects inside the state object. You could check if the item is an object.
$.each(state, function (i, sobj) {
if(typeof(sobj) == "object" && mid == sobj.user_id)
//some stuff;
});
You're iterating over the entire object like it's a collection of objects - meaning that trump in this case is an object, not a property of an object(state). You either need to iterate only over the collection of objects in state, or test that the object isn't null before you look for a property value:
if(typeof(sobj) != 'undefined'){
if(mid == sobj.user_id)
//some stuff;
}
I'm building a node scraper that uses cheerio to parse the DOM. This is more or a vanilla javascript question though. At one part of my scrape, I'm loading some content into a variable, then checking the variable's length, like so:
var theHref = $(obj.mainImg_select).attr('href');
if (theHref.length){
// do stuff
} else {
// do other stuff
}
This works just fine, until I came across a url for which $(obj.mainImg_select).attr('href') didn't exist. I assumed that my theHref.length check would account for this and skip through to the else: do other stuff statement, but instead I got:
TypeError: Cannot read property 'length' of undefined
What am I doing wrong here and how can I fix this?
You can check that theHref is defined by checking against undefined.
if (undefined !== theHref && theHref.length) {
// `theHref` is not undefined and has truthy property _length_
// do stuff
} else {
// do other stuff
}
If you want to also protect yourself against falsey values like null then check theHref is truthy, which is a little shorter
if (theHref && theHref.length) {
// `theHref` is truthy and has truthy property _length_
}
Why?
You asked why it happens, let's see:
The official language specificaion dictates a call to the internal [[GetValue]] method. Your .attr returns undefined and you're trying to access its length.
If Type(V) is not Reference, return V.
This is true, since undefined is not a reference (alongside null, number, string and boolean)
Let base be the result of calling GetBase(V).
This gets the undefined part of myVar.length .
If IsUnresolvableReference(V), throw a ReferenceError exception.
This is not true, since it is resolvable and it resolves to undefined.
If IsPropertyReference(V), then
This happens since it's a property reference with the . syntax.
Now it tries to convert undefined to a function which results in a TypeError.
There's a difference between an empty string "" and an undefined variable. You should be checking whether or not theHref contains a defined string, rather than its lenght:
if(theHref){
// ---
}
If you still want to check for the length, then do this:
if(theHref && theHref.length){
// ...
}
In addition to others' proposals, there is another option to handle that issue.
If your application should behave the same in case of lack of "href" attribute, as in case of it being empty, just replace this:
var theHref = $(obj.mainImg_select).attr('href');
with this:
var theHref = $(obj.mainImg_select).attr('href') || '';
which will treat empty string ('') as the default, if the attribute has not been found.
But it really depends, on how you want to handle undefined "href" attribute. This answer assumes you will want to handle it as if it was empty string.
If you aren't doing some kind of numeric comparison of the length property, it's better not to use it in the if statement, just do:
if(theHref){
// do stuff
}else{
// do other stuff
}
An empty (or undefined, as it is in this case) string will evaluate to false (just like a length of zero would.)
As has been discussed elsewhere, the .length property reference is failing because theHref is undefined. However, be aware of any solution which involves comparing theHref to undefined, which is not a keyword in JavaScript and can be redefined.
For a full discussion of checking for undefined variables, see Detecting an undefined object property and the first answer in particular.
You can simply check whether the element length is undefined or not just by using
var theHref = $(obj.mainImg_select).attr('href');
if (theHref){
//get the length here if the element is not undefined
elementLength = theHref.length
// do stuff
} else {
// do other stuff
}
Can not quite understand the reasoning for this. In the following code the localStorage of an item is alerted as undefined, but if I use an if(x==undefined) syntax it does not work. Can somebody explain what is the problem. Thank you.
alert(localStorage["x"]);
if(localStorage["x"]=="undefined"){alert("y");}
The top line alerts undefined
The bottom line does not alert y for me.
It doesn't contain the string "undefined", it contains a value of the type undefined:
if (localStorage["x"] == undefined) { alert("y"); }
The value undefined is possible to change in older browsers, so good practice is to check the type instead:
if (typeof localStorage["x"] == 'undefined') { alert("y"); }
Try:
if(typeof( localStorage["x"]) == 'undefined'){alert("y");}
OR
if( localStorage["x"] == undefined){alert("y");}
OR
if( !localStorage["x"] ){alert("y");}
The two ways of checking for something being undefined are:
typeof foo === "undefined"
and
foo === undefined
In the first case, it will be true if foo was never defined or the value of foo is undefined.
In the second case, it will only be true if foo was defined (otherwise it'll break) and its value is undefined.
Checking its value against the string "undefined" is not the same at all!
UPDATE:
When I said that if you try to perform an operation on an object literal's property that isn't defined, I guess I meant if it's undefined at all, and what I meant was something more like this:
obj["x"].toLowerCase()
// or
obj["x"]["y"]
where you are attempting to access/operate on something that is originally undefined. In this case, simply comparing in an if statement should be fine, because of the way object literals report the value...but is very different with normal Javascript variables.
With object literals, if a key (say "x") is not defined, then
obj["x"]
returns a value of undefined, so both the typeof and basic === undefined checks will work and be true.
The whole difference of not being defined or having a value of undefined is different with normal variables.
If you had:
var a;
// or
var a = undefined;
then both the typeof and basic === undefined checks I provided earlier would work and be true. But if you never even declared a, then only the typeof check would work and be true. The === undefined check would break.
Take a look at: http://jsfiddle.net/7npJx/
If you notice in the console, it says b is not defined and breaks the if statement.
Since you're basically looking at an object literal with localStorage, the way to distinguish whether an item is not defined or has a value of undefined is to use in first. So, you could use:
if (!("x" in localStorage)) {
to check if "x" is not a defined property at all, and:
else if (localStorage["x"] === undefined) {
to then check if it is defined but has a value of undefined. Then, using:
else {
would signify that localStorage["x"] is both defined and does not have the value undefined.
In your code though, it's okay to use the typeof or in checks (based on what you want to know) because of the way object literals report properties that aren't defined. Using the basic === undefined is also okay, but as Guffa pointed out, it's possible for the actual value of undefined to be overwritten and then wouldn't work in this comparison. When it comes to normal Javascript variables, typeof and === undefined checks aren't the same.
I've been writing JavaScript for quite a long time now, and I have never had a reason to use null. It seems that undefined is always preferable and serves the same purpose programmatically. What are some practical reasons to use null instead of undefined?
I don't really have an answer, but according to Nicholas C. Zakas, page 30 of his book "Professional JavaScript for Web Developers":
When defining a variable that is meant
to later hold an object, it is
advisable to initialize the variable
to null as opposed to anything else.
That way, you can explicitly check for the value null to determine if
the variable has been filled with an object reference at a later time
At the end of the day, because both null and undefined coerce to the same value (Boolean(undefined) === false && Boolean(null) === false), you can technically use either to get the job done. However, there is right way, IMO.
Leave the usage of undefined to the JavaScript compiler.
undefined is used to describe variables that do not point to a reference. It is something that the JS compiler will take care for you. At compile time the JS engine will set the value of all hoisted variables to undefined. As the engine steps through the code and values becomes available the engine will assign respective values to respective variables. For those variables for whom it did not find values, the variables would continue to maintain a reference to the primitive undefined.
Only use null if you explicitly want to denote the value of a variable as having "no value".
As #com2gz states: null is used to define something programmatically empty. undefined is meant to say that the reference is not existing. A null value has a defined reference to "nothing". If you are calling a non-existing property of an object, then you will get undefined. If I would make that property intentionally empty, then it must be null so you know that it's on purpose.
TLDR; Don't use the undefined primitive. It's a value that the JS compiler will automatically set for you when you declare variables without assignment or if you try to access properties of objects for which there is no reference. On the other hand, use null if and only if you intentionally want a variable to have "no value".
Sidebar: I, personally, avoid explicitly setting anything to undefined (and I haven't come across such a pattern in the many codebases/third party libs I've interacted with). Also, I rarely use null. The only times I use null is when I want to denote the value of an argument to a function as having no value, i.e.,:
function printArguments(a,b) {
console.log(a,b);
}
printArguments(null, " hello") // logs: null hello
null and undefined are essentially two different values that mean the same thing. The only difference is in the conventions of how you use them in your system. As some have mentioned, some people use null for meaning "no object" where you might sometimes get an object while undefined means that no object was expected (or that there was an error). My problem with that is its completely arbitrary, and totally unnecessary.
That said, there is one major difference - variables that aren't initialized (including function parameters where no argument was passed, among other things) are always undefined.
Which is why in my code I never use null unless something I don't control returns null (regex matching for example). The beauty of this is it simplifies things a lot. I never have to check if x === undefined || x === null, I can just check x === undefined. And if you're in the habit of using == or simply stuff like if(x) ... , stop it.
!x will evaluate to true for an empty string, 0, null, NaN - i.e. things you probably don't want. If you want to write javascript that isn't awful, always use triple equals === and never use null (use undefined instead). It'll make your life way easier.
undefined is where no notion of the thing exists; it has no type, and it's never been referenced before in that scope; null is where the thing is known to exist, but it has no value.
Everyone has their own way of coding and their own internal semantics, but over the years I have found this to be the most intuitive advice that I give people who ask this question: when in doubt, do what JavaScript does.
Let's say you are working with object properties like options for a jQuery plugin...ask yourself what value JavaScript gives a property that has yet to be defined -- the answer is undefined. So in this context, I would initialize these types of things with 'undefined' to be consistent with JavaScript (for variables, you can do var myVar; instead of var myVar = undefined;).
Now let's say you are doing DOM manipulation...what value does JavaScript assign to non-existent elements? The answer is null. This is the value I would initialize with if you are creating a placeholder variable that will later hold a reference to an element, document fragment, or similar that relates to the DOM.
If you're working with JSON, then a special case needs to be made: for undefined property values, you should either set them to "" or null because a value of undefined is not considered proper JSON format.
With this said, as a previous poster has expressed, if you find that you're initializing stuff with null or undefined more than once in a blue moon, then maybe you should reconsider how you go about coding your app.
You might adopt the convention suggested here, but there really is no good reason to. It is not used consistently enough to be meaningful.
In order to make the convention useful, you first must know that the called function follows the convention. Then you have to explicitly test the returned value and decide what to do. If you get undefined, you can assume that some kind of error occurred that the called function knew about. But if an error happened, and the function knew about it, and it is useful to send that out into the wider environment, why not use an error object? i.e. throw an error?
So at the end of the day, the convention is practically useless in anything other than very small programs in simple environments.
A few have said that it is ok to initialise objects to null. I just wanted to point out that destructuring argument defaults don't work with null. For example:
const test = ({ name } = {}) => {
console.log(name)
}
test() // logs undefined
test(null) // throws error
This requires performing null checks prior to calling the function which may happen often.
A useful property in null that undefined does not qualifies:
> null + 3
3
> undefined + 3
NaN
I use null when I want to 'turn off' a numeric value,
or to initialize some. My last use was manipulating css transform:
const transforms = { perspective : null, rotateX : null };
// if already set, increase, if not, set to x
runTimeFunction((x) => { trasforms.perspective += x; });
// still useful, as setting perspective to 0 is different than turning it off
runTimeFunction2((x) => { transforms.perspective = null; });
// toCss will check for 'null' values and not set then at all
runTimeFunction3(() => { el.style.transform = toCss(transforms); });
Not sure if I should use this property thought...
DOM nodes and elements are not undefined, but may be null.
The nextSibling of the last child of an element is null.
The previousSibling of the first child is null.
A document.getElementById reference is null if the element does not exist in the document.
But in none of these cases is the value undefined; there just is no node there.
Unknown variable: undefined.
Known variable yet no value: null.
You receive an object from a server, server_object.
You reference server_object.errj. It tells you it’s undefined. That means it doesn’t know what that is.
Now you reference server_object.err. It tells you it’s null. That means you’re referencing a correct variable but it’s empty; therefore no error.
The problem is when you declare a variable name without a value (var hello) js declares that as undefined: this variable doesn’t exist; whereas programmers mostly mean: “I’ve not given it a value yet”, the definition of null.
So the default behavior of a programmer—declaring a variable without a value as nothing—is at odds with js—declaring it as not existing. And besides, !undefined and !null are both true so most programmers treat them as equivalent.
You could of course ensure you always do var hello = null but most won’t litter their code as such to ensure type sanity in a deliberately loosely-typed language, when they and the ! operator treat both undefined and null as equivalent.
In JavaScript, the value null represents the intentional absence of any object value. null expresses a lack of identification, indicating that a variable points to no object.
The global undefined property represents the primitive value undefined.
undefined is a primitive value automatically assigned to variables.
undefined is meant to say that the reference is not existing.
I completely disagree that usage null or undefined is unnecessary.
undefined is thing which keeping alive whole prototype chaining process.
So compiler only with null can't check if this property just equal to null, or its not defined in endpoint prototype. In other dynamic typed languages(f.e. Python) it throws exception if you want access to not defined property, but for prototype-based languages compiler should also check parent prototypes and here are the place when undefined need most.
Whole meaning of using null is just bind variable or property with object which is singleton and have meaning of emptiness,and also null usage have performance purposes. This 2 code have difference execution time.
var p1 = function(){this.value = 1};
var big_array = new Array(100000000).fill(1).map((x, index)=>{
p = new p1();
if(index > 50000000){
p.x = "some_string";
}
return p;
});
big_array.reduce((sum, p)=> sum + p.value, 0)
var p2 = function(){this.value = 1, p.x = null};
var big_array = new Array(100000000).fill(1).map((x, index)=>{
p = new p2();
if(index > 50000000){
p.x = "some_string";
}
return p;
});
big_array.reduce((sum, p)=> sum + p.value, 0)
I'm working through this exact question right now, and looking at the following philosophy:
Any function that is intended to return a result should return null if it fails to find a result
Any function that is NOT intended to return a result implicitly returns undefined.
For me, this question is significant because anyone calling a function that returns a result should have no question as to whether to test for undefined vs null.
This answer does not attempt to address:
Property values of null vs undefined
Variables within your functions being null vs undefined
In my opinion, variables are your own business and not a part of your API, and properties in any OO system are defined and therefore should be defined with value different from what they would be if not defined (null for defined, undefined is what you get when accessing something that is not in your object).
Here's a reason: var undefined = 1 is legal javascript, but var null = 1 is a syntax error. The difference is that null is a language keyword, while undefined is, for some reason, not.
If your code relies on comparisons to undefined as if it's a keyword (if (foo == undefined) -- a very easy mistake to make) that only works because nobody has defined a variable with that name. All that code is vulnerable to someone accidentally or maliciously defining a global variable with that name. Of course, we all know that accidentally defining a global variable is totally impossible in javascript...
Just wanna add that with usage of certain javascript libraries, null and undefined can have unintended consequences.
For example, lodash's get function, which accepts a default value as a 3rd argument:
const user = {
address: {
block: null,
unit: undefined,
}
}
console.log(_.get(user, 'address.block', 'Default Value')) // prints null
console.log(_.get(user, 'address.unit', 'Default Value')) // prints 'Default Value'
console.log(_.get(user, 'address.postalCode', 'Default Value')) // prints 'Default Value'
Another example: If you use defaultProps in React, if a property is passed null, default props are not used because null is interpreted as a defined value.
e.g.
class MyComponent extends React.Component {
static defaultProps = {
callback: () => {console.log('COMPONENT MOUNTED')},
}
componentDidMount() {
this.props.callback();
}
}
//in some other component
<MyComponent /> // Console WILL print "COMPONENT MOUNTED"
<MyComponent callback={null}/> // Console will NOT print "COMPONENT MOUNTED"
<MyComponent callback={undefined}/> // Console WILL print "COMPONENT MOUNTED"
There are already some good answers here but not the one that I was looking for. null and undefined both "technically" do the same thing in terms of both being falsy, but when I read through code and I see a "null" then I'm expecting that it's a user defined null, something was explicitly set to contain no value, if I read through code and see "undefined" then I assume that it's code that was never initialized or assigned by anything. In this way code can communicate to you whether something was caused by uninitialized stuff or null values. Because of that you really shouldn't assign "undefined" manually to something otherwise it messes with the way you (or another developer) can read code. If another developer sees "undefined" they're not going to intuitively assume it's you who made it undefined, they're going to assume it's not been initialized when in fact it was. For me this is the biggest deal, when I read code I want to see what it's telling me, I don't want to guess and figure out if stuff has "actually" been initialized.
Not even to mention that using them in typescript means two different things. Using:
interface Example {
name?: string
}
Means that name can be undefined or a string, but it can't be null. If you want it null you have to explicitly use:
interface Example {
name: string | null
}
And even then you'll be forced to initialize it at least with "null".
That's of course only true if you're using "strictNullChecks": true in tsconfig.json.
Based on a recent breakage we ran into, the example below shows why I prefer to use undefined over null, unless there is a specific reason to do otherwise:
function myfunc (myArg) {
if (typeof myArg === 'string') {
console.log('a', myArg);
} else if (typeof abc === 'object') {
console.log('b', myArg);
if (myArg.id) {
console.log('myArg has an id');
} else {
console.log('myArg has an id');
}
} else {
console.log('no value');
}
}
The following values will play nicely:
'abc'
{}
undefined
{ id: 'xyz' }
On the other hand the assumption of null and undefined being equivalent here breaks the code. The reason being is that null is of type of object, where as undefined is of type undefined. So here the code breaks because you can't test for a member on null.
I have seen a large number of cases with code of similar appearance, where null is just asking for problems:
if (typeof myvar === 'string') {
console.log(myvar);
} else if (typeof myvar === 'object') {
console.log(myvar.id);
}
The fix here would be to explicitly test for null:
if (typeof myvar === 'string') {
console.log(myvar);
} else if (myvar !== null && typeof myvar === 'object') {
console.log(myvar.id);
}
My attitude is to code for the weaknesses of a language and the typical behaviours of programmers of that language, hence the philosophy here of going with 'undefined' bey default.
To write simple code you need to keep complexity and variation down. When a variable or a property on an object does not have a value it is undefined , and for a value to be null you need to assign it a null value.
Undeclared vs Null
null is both an Object "type" and one of the 7 unique primitive value types called null
undefined is both a global scope property and type called undefined and one of the 7 unique primitive value types called undefined (window.undefined) .
It is the primitive types we use as values we are interested in.
In the case of null, as a value type it means an empty value has been assigned to a variable, but the variable type (Number, String, etc) is still defined. It just has no value. That is what null means. It means a variable has an empty value but it is still a value. It also reinitializes the variable with some kind of value, but is not undefined as a type.
undefined is a special case. When you declare a variable (or use a missing value not yet declared) it is of type undefined, as the browser does not know what type of data has been assigned to it yet. If the variable is declared but not assigned a value is is assigned the primitive calue undefined by default prior to assigning a value, and implies the variable does not exist or exists but has no value assigned.
Like null, undefined is also a primitive value type. But unlike null it means the variable does not exist, where null means the value does not exist. That is why its always better to check if the variable exists and has been assigned a variable using undefined before checking if the value is null or empty. undefined implies no variable or object exists in the compilation at all. The variable has either not been declared or declared with a missing value so not initialized. So checking for undefined is a very good way to avoid many types of errors in JavaScript and supersedes null.
That is why I would not rely on "truthy" checks for true/false with null and undefined, even though they will both return a false response, as undefined implies an additional step for missing feature, object, or variable, not just a true/false check. It implies something more. If you have a missing undeclared variable, truthy statements will trigger an ERROR!
Let's look at undefined first:
//var check1;// variable doesnt even exist so not assigned to "undefined"
var check2;// variable declared but not initialized so assigned "undefined"
var check3 = 'hello world';// variable has a value so not undefined
console.log('What is undefined?');
//console.log(check1 === undefined);// ERROR! check1 does not exist yet so not assigned undefined!
console.log(check2 === undefined);// True
console.log(check3 === undefined);// False
console.log(typeof check1 === 'undefined');// True - stops the ERROR!
console.log(typeof check2 === 'undefined');// True
console.log(typeof check3 === 'undefined');// False
As you can see undeclared variables, or declared but not initialized, both are assigned a type of undefined. Notice declared variables that are not initialized are assigned a value of undefined, the primitive value type but variables that do not exist are undefined types.
null has nothing to do with missing variables or variables not yet assigned values, as null is still a value. So anything with a null is already declared and initialized. Also notice a variable assigned a null value is actually an object type unlike undefined types. For example...
var check4 = null;
var check5 = 'hello world';
console.log('What is null?');
console.log(check4 === undefined);// False
console.log(check5 === undefined);// False
console.log(typeof check4 === 'undefined');// False
console.log(typeof check5 === 'undefined');// False
console.log(typeof check4);// return 'object'
console.log(typeof check5);// return 'string'
As you can see each act differently and yet both are primitive values you can assign any variable. Just understand they represent different states of variables and objects.