CODEWITHSUNDEEP
javascript
chris Frisina
chris Frisina

Reputation: 19688

General constructor for all JS types

If I am trying to declare a variable and I need to provide a default case because it won't let me create the key without a value, I thought I could make the same basic value for all empty variables, but it still be of the type I want it to be when I actually assign it the value.

var x = "";
var y = 0;
var z = null;

I know that JS is pretty good about knowing which type (except those crazy situations between strings and numbers), but would null be able to be over-written by all types, and then that variable would be of that type?

so if we had var x = ""; then x is a String already. Obviously if I did var x = "something" then it would still be a String, or if I did var x = 12 then it would change the type and the value.

What is the type if it is assigned null?

So what is your suggestion for a placeholder value for a variable that just needs to be created?

Upvotes: 0

Views: 67

Answers (4)

Halcyon
Halcyon

Reputation: 57703

A default value is not the same as no value. Sometimes it's important to be able to make that distinction.

Since you do need some way to detect if you have a value you can use a value that indicates that the value is undefined, like: undefined. This does mean that your dictionary is initially empty.

Depending on your data types you can also use null. Of course if null is a possible value you can't use it. If you have no safe undefined value you can have another property say something about the value, like is value X set: yes/no (this is one step in the direction of a schema / meta data).

A tip I can give you is that if you have data, with strongly typed data types, work on a schema language to describe the data. You will get the undefined value detection basically for free.

Upvotes: 1

shennan
shennan

Reputation: 11666

null = Object;

undefined = no type specified

In Javascript, everything is an Object of some form or other, unless it has not been associated with any data, in which case it is undefined.

So you probably want something like:

var defaults = {
  beats: "",
  numberOfBeats: 0,
  representation: undefined
};

Then in your code you could query whether the representation index has any data in it by:

if(typeof defaults['representation'] === "undefined"){

    // representation has not yet been defined

}

Upvotes: 1

jcreamer898
jcreamer898

Reputation: 8189

var x;
x = 6;
typeof x // 'number'

x = '6';
typeof x // 'string'

x = null;
typeof x // 'object'

x = {};
typeof x // 'object'

x = 3;
typeof x // 'number'

Upvotes: 1

Alan
Alan

Reputation: 46873

The typeof null is "object"

And it's a common mistake that novice js programmers make to assume that undefined and null are the same (they're not).

Upvotes: 1

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