Declaring javascript variables as specific types [closed]

Question

The title says it all, but I will provide more clarification:

After seeing many samples of javascript where all variables are declared as type var, and seeing support for other datatypes, why aren't variables of a specific datatype declared as such? Meaning, why isn't this:

string hello = 'Hello, World'

used instead of

var hello = 'Hello, World'

Looking at sites like OReilly Javascript shows that there are reserved words for other types. Again, why aren't they used? Wouldn't it make lines like this: typeof(variable)==='string'; no longer needed?

Javascript is an untyped language. stackoverflow.com/questions/964910/… — Jonas G. Drange
– Jonas G. Drange, Commented Jan 7, 2013 at 19:33
@JonasG.Drange ECMAScript is a dynamically typed language. (Values still have types and thus can't be un-typed language.) — user166390
– user166390, Commented Jan 7, 2013 at 19:42
@pst read the linked answer carefully. There is a deliberate abuse of the language "untyped" to be a shorthand for "no static types." I disagree with that abuse, but that's another issue. — Matt Ball
– Matt Ball, Commented Jan 7, 2013 at 19:44

Matt Ball · Accepted Answer · 2013-01-07 19:33:17Z

75

Quite simply, JavaScript variables do not have types. The values have types.

The language permits us to write code like this:

var foo = 42;
foo = 'the answer';
foo = function () {};

So it would be pointless to specify the type in a variable declaration, because the type is dictated by the variable's value. This fairly common in "dynamic" languages.

answered Jan 7, 2013 at 19:33

Matt Ball

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6 Comments

CBredlow Over a year ago

So it's not possible to declare a type beforehand?

AVIK DUTTA Over a year ago

what if i do var speed : float=10.00;

Matt Ball Over a year ago

@AVIKDUTTA that's syntactically invalid JavaScript.

M3579 Over a year ago

@AVIKDUTTA That is valid UnityScript (the version of JavaScript that the game engine Unity uses), but not JavaScript.

klewis Over a year ago

@MattBall it appears you are saying there is no way to always expect a variable value to always be a certain type in JavaScript? is that correct? Is this something that we "can" do with TypeScript?

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