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'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.
I am wondering if this is possible in JavaScript, I want to have an Object which could contain dynamic properties.
Give an example:
function MyObject()
{
}
var myobj = new MyObject();
myobj.property1 = "something";
alert(myobj.property1); // something
alert(myobj.property2); // this should never report error, instead the property should be evaluated to null, as it has never been set.
Is there any way to intercept property calls in JavaScript so I can proactively set a no-value property as null?
Thanks.
This is about as close as you can get to achieving your goal.
Code:
var obj = {};
alert("prop: " + obj.prop);
obj.prop = "something";
alert("prop: " + obj.prop);
delete obj.prop;
alert("prop: " + obj.prop);
Behavior:
Alert: "prop: undefined"
Alert: "prop: something"
Alert: "prop: undefined"
'Proxy' can do that
var myobj = new Object();
var handler = {
get:function (obj, name, proxyed){
if(obj[name] !== undefined) // if obj[name] exist
return obj[name]; // then return obj[name]
return null; // if obj[name] is not exist then return null;
}
};
var obj = new Proxy(myobj, handler);
obj.property1 = "something";
alert(myobj.property1); // something
alert(myobj.property2); // undefined
alert(obj.property1); // something
alert(obj.property2); // null
Yes, but only in version 2.0 and higher. The exact syntax is still TBD but it's looking like it'll be get * () {...} for object literals at least.
Nope. JavaScript is not Smalltalk.
There is no way to intercept direct property accesses in JavaScript. When a property is retrieved that hasn't been set than the result will be undefined. Although null and undefined are usually considered to be the same thing they are in fact different entities.
In JavaScript undefined means no value and null means a value of null. In some cases you can mix undefined and null. For example, when using the == operator they are equivalent ((null == undefined) === true). Using the non-coercing operator, ===, they are different ((null === undefined) === false).
You can use this to your advantage. While most people will claim that you should use the non-coercing equality operator (===) it's mostly safe to put null and undefined in the same bucket, in less of course you actually care about the difference between the two. Where it gets tricky is that undefined is a property of the global object and can therefore be assigned a new value.
If someone were to say undefined = 'donkey' then null == undefined would start to return false. In practice this is almost never a problem since most people aren't foolish enough to reassign the value of undefined.
So, in a roundabout sort of way, you don't need to trap property accesses to return null for properties that have not been set so long as you compare the result against null using ==.
No unless you are manipulating an object controlled by an NPAPI plugin in which case you could implement the intended behavior.
In other words, through an NPAPI plugin, you could implement the behavior you are looking for.
Check out javascript prototypes. I think that will give you at least some of what you are looking for. Just google up "javascript prototype".
In your example the second alert will not generate an error. It will just alert undefined. Accessing properties of properties will generate an error:
myobj.property2.property3