Why How to Declare a Variable Without an Initial Value

Declare a variable without an initial value

The error is correct, you can only do that on an abstract class or trait. The tutorial might be assuming that you are writing that code inside of an abstract class.

It is possible to initialize variables to some default value:

var i: Int = _
var s: String = _

But that's essentially the same as:

var i: Int = 0
var s: String = null

Why is it possible to declare a variable without an initial value?

There are many different reasons for many different languages.

MEMORY

When you declare a variable, you want some memory to hold the variable in. This involves asking the kernel of the operating system for memory, or some kind of monitoring program which keeps track of memory. In short, this can be an expensive operation.Hence, in many cases, it is desirable to allocate all the memory required for the object at the same time, and then assign whatever value has to be assigned to it later. This way, you can increase the performance of the program in the critical parts. This use case is common enough that a feature allowing declaration without initialization is allowed. However, good practices assert that in all other cases you should initialize the variable while assigning.

Think of the memory allocation as a bureaucracy. There is too much paper work. So, if you know you are going to use a large amount of memory later, you ask for a large amount of memory upfront in one single transaction, rather than asking the kernel each next time.

EXPENSIVE INITIALIZATION

This point is very similar to the above point. Suppose you have a 1 million times 1 million array. Initializing such an array is an expensive procedure. To do so with defaults would be stupidity, and hence, such a feature, where memory is allocated and then used as needed.

In here, its like you are buying a huge amount of lego blocks to construct something, but you want to buy them in shapes of the default spiderman. The shopkeeper or you would have to extra hard to get them in shapes of spiderman when you are anyway going to reshape them later.

Is it possible only to declare a variable without assigning any value in Python?

Why not just do this:

var = None

Python is dynamic, so you don't need to declare things; they exist automatically in the first scope where they're assigned. So, all you need is a regular old assignment statement as above.

This is nice, because you'll never end up with an uninitialized variable. But be careful -- this doesn't mean that you won't end up with incorrectly initialized variables. If you init something to None, make sure that's what you really want, and assign something more meaningful if you can.

Declaring variables without a value

If you want to define the variables as properties of GlobalVars you have to explicitly assign undefined to them

GlobalVars: {
     globalVarA: "foo",
     globalVarB: "bar",
     globalVarC: undefined
  },

Your code contained invalid notation, when using object literal notation you must specify a value for each variable. This is unlike other notations like XML or formParams where you can declare a attribute/property without setting it.

Note, that in my solution undefined variables do get a default value - that value is just the primitive value undefined which is what is returned by default for variables that were not defined.

However, your variable does get defined if you do globalVarC: undefined

GlobalVars.globalVarC;//undefined
GlobalVars.globalVarX;//undefined
GlobalVars.hasOwnProperty("globalVarC");//true
GlobalVars.hasOwnProperty("globalVarX");//false

From the spec:

4.3.9 undefined value:

primitive value used when a variable has not been assigned a value.

Is it necessary to Initialize / Declare variable in PHP?

PHP does not require it, but it is a good practice to always initialize your variables.

If you don't initialize your variables with a default value, the PHP engine will do a type cast depending on how you are using the variable. This sometimes will lead to unexpected behaviour.

So in short, in my opinion, always set a default value for your variables.

P.S.
In your case the value should be set to "" (empty string), instead of null, since you are using it to concatenate other strings.

Edit

As others (@n-dru) have noted, if you don't set a default value a notice will be generated.

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