It can be described as the absence of a string instance. Managed bean elements represented as a JavaServer Faces text component such as inputText are initialized with the value of the empty string by the JavaServer Faces implementation.
Validating these strings can be an issue when user input for such fields is not required. Consider the following example, in which the string testString is a bean variable that will be set using input entered by the user. Your Dart program has a whole universe of types in it: primitive types like int and String , collection types like List , and all of the classes and types you and the packages you use define.
Before null safety, the static type system allowed the value null to flow into expressions of any of those types. In type theory lingo, the Null type was treated as a subtype of all types:. The set of operations—getters, setters, methods, and operators—allowed on some expression are defined by its type. If the type is List , you can call. Allowing null to flow into an expression of some other type means any of those operations can fail. Null safety eliminates that problem at the root by changing the type hierarchy.
Instead, the type hierarchy looks like this:. Since Null is no longer a subtype, no type except the special Null class permits the value null. If you have a variable of type String , it will always contain a string. But null is useful, so we still need a way to handle it. Optional parameters are a good illustrative case. Consider this null safe Dart code:.
Here, we want to allow the dairy parameter to accept any string, or the value null , but nothing else. To express that, we give dairy a nullable type by slapping? Under the hood, this is essentially defining a union of the underlying type and the Null type. So String? If you have an expression with a nullable type, what can you do with the result? Since our principle is safe by default, the answer is not much.
This would crash if we let you run it. The only methods and properties we can safely let you access are ones defined by both the underlying type and the Null class. How do they interact with non-nullable types?
If a function accepts String? We model this by making every nullable type a supertype of its underlying type. You can also safely pass null to something expecting a nullable type, so Null is also a subtype of every nullable type:. But going the other direction and passing a nullable type to something expecting the underlying non-nullable type is unsafe.
Code that expects a String may call String methods on the value. If you pass a String? However, Dart has always had this thing called implicit downcasts. If you, for example, pass a value of type Object to a function expecting an String , the type checker allows it:. To maintain soundness, the compiler silently inserts an as String cast on the argument to requireStringNotObject. That cast could fail and throw an exception at runtime, but at compile time, Dart says this is OK.
Since non-nullable types are modeled as subtypes of nullable types, implicit downcasts would let you pass a String? Allowing that would violate our goal of being safe by default. So with null safety we are removing implicit downcasts entirely. This makes the call to requireStringNotNull produce a compile error, which is what you want. But it also means all implicit downcasts become compile errors, including the call to requireStringNotObject. We think this is an overall good change.
Our impression is that most users never liked implicit downcasts. In particular, you may have been burned by this before:. Spot the bug? This program compiles but then throws an exception at runtime when it tries to cast that Iterable to the List type that filterEvens declares it returns.
With the removal of implicit downcasts, this becomes a compile error. Where were we? There is a region of non-nullable types. Those types let you access all of the interesting methods, but can never ever contain null. And then there is a parallel family of all of the corresponding nullable types.
We let values flow from the non-nullable side to the nullable side because doing so is safe, but not the other direction. That seems like nullable types are basically useless. This section is a little esoteric. Imagine all the types in your program with edges between ones that are subtypes and supertypes of each other. If you were to draw it, like the diagrams in this doc, it would form a huge directed graph with supertypes like Object near the top and leaf classes like your own types near the bottom.
If that directed graph comes to a point at the top where there is a single type that is the supertype directly or indirectly , that type is called the top type. Likewise, if there is a weird type at that bottom that is a subtype of every type, you have a bottom type.
In this case, your directed graph is a lattice. Since Object is non-nullable now, it is no longer a top type. Null is not a subtype of it. Dart has no named top type. If you need a top type, you want Object? Likewise, Null is no longer the bottom type.
If it was, everything would still be nullable. If you want to indicate that you allow a value of any type, use Object? On the rare occasion that you need a bottom type, use Never instead of Null.
We divided the universe of types into nullable and non-nullable halves. In order to maintain soundness and our principle that you can never get a null reference error at runtime unless you ask for it, we need to guarantee that null never appears in any type on the non-nullable side. Getting rid of implicit downcasts and removing Null as a bottom type covers all of the main places that types flow through a program across assignments and from arguments into parameters on function calls.
The main remaining places where null can sneak in are when a variable first comes into being and when you leave a function. So there are some additional compile errors:. If a function has a non-nullable return type, then every path through the function must reach a return statement that returns a value.
Before null safety, Dart was pretty lax about missing returns. For example:. If you analyzed this, you got a gentle hint that maybe you forgot a return, but if not, no big deal. With sound non-nullable types, this program is flat out wrong and unsafe. As long as they all return something, it is satisfied. The analysis is pretty smart, so even this function is OK:.
So we have to tighten things up for non-nullable variables:. Top level variable and static field declarations must have an initializer. The only safe option is to require the declaration itself to have an initializing expression that produces a value of the right type:. Here are the examples:. Local variables are the most flexible case. This is perfectly fine:. The rule is only that a local variable must be definitely assigned before it is used.
We get to rely on the new flow analysis I alluded to for this as well. Optional parameters must have a default value. So, if you want a parameter to be optional, you need to either make it nullable or specify a valid non- null default value. Also, remember that these only apply to non-nullable variables. You can always make the type nullable and then get the default initialization to null.
Even so, the rules do cause friction. Fortunately, we have a suite of new language features to lubricate the most common patterns where these new limitations slow you down. Control flow analysis has been around in compilers for years.
Dart already has a dash of flow analysis in the form of type promotion :. Enterprise search for employees to quickly find company information. Detect, investigate, and respond to online threats to help protect your business.
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Cloud-native relational database with unlimited scale and Serverless, minimal downtime migrations to Cloud SQL. Note that this is an attempt at humour, before you flame me for misunderstanding the question. Show 2 more comments. Active Oldest Votes. Improve this answer. Lodewijk Bogaards Today, it is rationalized that null is considered a placeholder for an object, even though, technically, it is a primitive value. The comment from Nir O. If I want to have a variable that has no value in the beginning, I write " Wolfgang Adamec, error-free programming is not about mistypes.
While undefined means. Show 13 more comments. 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. BSMP 4, 8 8 gold badges 33 33 silver badges 43 43 bronze badges. Sebastian Norr Sebastian Norr 4, 2 2 gold badges 8 8 silver badges 16 16 bronze badges. Could you include the attribution for the image source, please?
Vega Unfortunately no, I don't remember where I got it from other than somewhere on imgur. Not even the embedded link here gives any clue of who posted this version, so I can't really search for it either. SebastianNorr "I don't remember where I got it from other than somewhere on imgur. Erowlin It is closer to being null because I remember that the image comes from imgur, so it isn't totally unknown.
The undefined would be more like "I have no idea where it came from". I picked this from here The undefined value is a primitive value used when a variable has not been assigned a value. However, if you append a blank string to it then suddenly it'll appear: var s; WScript. Echo s ; WScript. You can see this by using typeof which returns a string representing the general type of a variable: var a; WScript. Echo typeof b ; Running the above script will result in the following output: undefined object Regardless of their being different types, they will still act the same if you try to access a member of either one, e.
Victor 4, 10 10 gold badges 48 48 silver badges 83 83 bronze badges. Really a good answer. Hence it returned "true". It's worth noting that while this comment was true in '11, with the advent of optional function params, emergence of type-checking systems like Flow, and pervasiveness of React all of which treat undefined and null very differently , the old wisdom of generally using null rather than undefined no longer holds so strictly.
Add a comment. Dovlet Mamenov 6 6 silver badges 20 20 bronze badges. Mina Gabriel Mina Gabriel Both undefined and null are data types and values — danwellman. The fact that typeof null returns object is a well known and documented bug in early versions of ECMAScript that has remained for backwards-compatibility.
So please, show some search effort before commenting on down-votes — danwellman. Definitions contradict: "absence of value" vs "has not been assigned a value". Isn't it the same? I disagree with this answer.
Null and undefined are both distinct datatypes. For all other objects user-defined objects , it always returns '[object Object]' If that last part is the case the stringified version of the variable being '[object Object]' and the parameter returnConstructorBoolean is true , it will try to get the name of the constructor by toString -ing it and extracting the name from there.
If the constructor can't be reached, 'object' is returned as usual. Gust van de Wal 4, 1 1 gold badge 20 20 silver badges 43 43 bronze badges. Arup Hore Arup Hore 1, 14 14 silver badges 9 9 bronze badges. Dmitry Sychov Dmitry Sychov 7 7 silver badges 16 16 bronze badges. That's a really pithy definition, but it's not really accurate - for the reason you give. A defined variable starts with the value undefined. Kevin Kevin 1 1 gold badge 7 7 silver badges 19 19 bronze badges.
I really like this description. This is what I think I'm going to use as well. If I receive a null I know the value was set to null on purpose — Stevenfowler I'll explain undefined , null and Uncaught ReferenceError : 1 - Uncaught ReferenceError : variable has not been declared in your script, there is no reference to this varaible 2 - undefined : Variable declared but does not initialised 3 - null : Variable declared and is an empty value.
Echo null ; Oops! Consider the following: WScript. Crowder k gold badges silver badges bronze badges. Richard H Richard H It is important to note, that while undefined may mean that a variable has not been declared, but does not guarantee that. A variable can be declared as var thing; and it will be equal to undefined.
So what are some of the JavaScript-specific issues that arise due to this philosophical basis? NaN Using a variable that has never been declared and receiving a ReferenceError Using a let or const defined local variable in its temporal dead zone and receiving a ReferenceError Empty cells in sparse arrays. Ray Toal Ray Toal Best answer! Thank you, and yes, that misconception that something is undefined only if it has not been declared or has not yet been given a value is so rampant and it is really hard to get across to people though I keep trying.
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