Question

Just wondering what’s the meaning of “:” (colon symbol) on this Javascript code below?

var switchToTarget : Transform;

Thanks,
Gino

Answer 1

Edit: Reading more about Unity, they have created a really custom implementation of JavaScript⁽¹⁾ for their scripting engine, which is compiled and it has a lot of strongly typing features, it looks like ActionScript/ES4, but it isn’t, the language is called UnityScript.

The colon is used by this implementation to denote the type of an identifier, e.g.:

class Person{
   var name : String;
   function Person(n : String){
      name = n;
   }
   function kiss(p : Person){
      Debug.Log(name + " kissed " +  p.name + "!");
   }
}

See also:

• UnityScript Reference
• Head First into Unity with JavaScript
• Scripting Overview
• Unity Answers

---

The code you posted is not valid ECMAScript 3, (which is the most widely implemented standard), that will simply give you a SyntaxError.

The colon symbol in JavaScript has only a few usages:

1. The object literal syntax:

   var obj = { foo: 'bar' };
   

2. The conditional operator:

   var test = condition ? 'foo' : 'bar';
   

3. Labeled statements:

   loop1: while (true) {
     while (true) {
       break loop1; // stop outer loop
     }
   }
   

4. Case and default clauses of the switch statement:

   switch (value) {
     case "foo":
       //..
     break;
     default:
       //..
     break;
   }
   

5. It can appear on RegExp literals:

   var re = /(?:)/; // non-capturing group...