Undefined typically refers to something which has not yet been assigned a value (yet). Null refers to something which definitively has no value. In that case, I would recommend returning a null. Note that a function with no specified return value implicitly returns undefined.
From the ECMAScript2015 spec
4.3.10 undefined value
primitive value used when a variable has not been assigned a value
4.3.12 null value
primitive value that represents the intentional absence of any object value
http://www.ecma-international.org/ecma-262/6.0/#sec-terms-and-definitions-undefined-type
Further reading:
When is null or undefined used in JavaScript?
Answer from chiliNUT on Stack OverflowUndefined typically refers to something which has not yet been assigned a value (yet). Null refers to something which definitively has no value. In that case, I would recommend returning a null. Note that a function with no specified return value implicitly returns undefined.
From the ECMAScript2015 spec
4.3.10 undefined value
primitive value used when a variable has not been assigned a value
4.3.12 null value
primitive value that represents the intentional absence of any object value
http://www.ecma-international.org/ecma-262/6.0/#sec-terms-and-definitions-undefined-type
Further reading:
When is null or undefined used in JavaScript?
I will give you my personal opinionated way of choosing between the two.
My simple question is: could the value, given another input/state/context be defined to something?
If the answer is yes then use null else use undefined. More generally any function returning an object should return null when the intended object does not exist. Because it could exist given another input/state/context.
null represents the absence of value for a given input/state/context. It implicitly means that the concept of the value itself exist in the context of your application but may be absent.
In your example the concept of a next card exists but the card itself may not exist. null should be used.
undefined implicitly represents the absence of meaning of that value in your application's context. For example, if I manipulate a user object with a given set of properties and I try to access the property pikatchu. The value of this property should be set to undefined because in my context it doesn't make any sense to have such a property.
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Please consider the following:
var myFruits = ['Banana', 'Apple', 'Strawberry'];// SOME CODING// SOME CODINGmyFruits = undefined; // Is this better?myFruits = null; // or is this better?
Further question, what is the distinction between the two? Is there any cases where only null is used or undefined is used? Thanks.
undefined means a variable has been declared but has not yet been assigned a value :
var testVar;
console.log(testVar); //shows undefined
console.log(typeof testVar); //shows undefinednull is an assignment value. It can be assigned to a variable as a representation of no value :
var testVar = null;
console.log(testVar); //shows null
console.log(typeof testVar); //shows objectFrom the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.
Proof :
console.log(null === undefined) // false (not the same type)
console.log(null == undefined) // true (but the "same value")
console.log(null === null) // true (both type and value are the same)and
null = 'value' // Uncaught SyntaxError: invalid assignment left-hand side
undefined = 'value' // 'value'
The difference can be explained with toilet tissue holder:
A non-zero value is like a holder with roll of toilet tissue and there's tissue still on the tube.
A zero value is like a holder with an empty toilet tissue tube.
A null value is like a holder that doesn't even have a tissue tube.
An undefined value is similar to the holder itself being missing.
Since switching to TypeScript I have been using a lot of optional properties, for example:
type store = {
currentUserId?: string
}
function logout () {
store.currentUserId = undefined
}However my coworkers and I have been discussing whether null is a more appropriate type instead of undefined, like this:
type store = {
currentUserId: string | null
}
function logout () {
store.currentUserId = null
}It seems like the use of undefined in TypeScript differs slightly from in Javascript.
Do you guys/girls use undefined or null more often? And, which of the examples above do you think is better?
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
nullas opposed to anything else. That way, you can explicitly check for the valuenullto 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
undefinedto the JavaScript compiler.undefinedis 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 toundefined. 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 primitiveundefined.Only use null if you explicitly want to denote the value of a variable as having "no value".
As @com2gz states:
nullis used to define something programmatically empty.undefinedis meant to say that the reference is not existing. Anullvalue has a defined reference to "nothing". If you are calling a non-existing property of an object, then you will getundefined. If I would make that property intentionally empty, then it must benullso 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