UnityScript versus JavaScript

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Though many in the Unity development community (and even in the Unity corporation) refer to UnityScript and JavaScript as if they were equivalent or interchangeable, the two are very different languages. Though they resemble each other syntactically, they have very different semantics.

While "JavaScript" is merely a generic name and could refer to any one of many implementations of the ECMAScript specification, Unity's ".js" language doesn't even come close to conforming to that specification. Most JavaScript libraries you can find will probably not work just by copying them into Unity. If anything, the language is most similar to Microsoft's JScript.NET, although it is not quite identical.

This page attempts to explain the differences between JavaScript (ECMAScript) and UnityScript as succinctly and clearly as possible. If you have any suggestions, feel free to add them to the author's talk page.


JavaScript is class-free

JavaScript has no classes. This is because it's a prototypal language, and not a classical one. Inheritance happens with [more dynamic] objects, rather than [unchanging] classes.

function Machine(x) {
   this.kind = ["bulldozer", "lathe", "car"][x];
var c = new Machine(2);
print(typeof c.announce); // "undefined"
Machine.prototype.announce = function() {
   print("I am a "+this.kind+".");
print(typeof c.announce); // "function"
c.announce(); // prints "I am a car."

As shown above, in JavaScript, a function can create an object, when called with the new keyword. After that happens, the prototype (template) object Machine can be extended to provide additional functionality, and all class instances, past and future, are affected by this extension.

UnityScript has classes, and functions cannot create objects as in JavaScript. Extension is therefore impossible. However, there is the added benefit of being an easier-to-read, more familiar (to most) language.

class Machine {
   var kind : String;
   function Machine(x : int) {
      this.kind = ["bulldozer", "lathe", "car"][x];
   function announce() {
      print("I am a "+this.kind+".");
print(typeof Machine.prototype); // undefined
var c = new Machine(2);
c.announce(); // prints  "I am a car."

One variable declaration at a time

JavaScript supports multiple variable declarations in one var statement.

var x = 3, y = 4;

UnityScript does not.

Assignment cannot be an expression

In JavaScript, assignment is treated as an expression.

var x = 3; // x is 3
var y = (x=x+2); // x is 5, y is 5

In Unity, assignment is always a statement, so this must be broken up into steps.

var x = 3; // x is 3
  // var y = (x=x+2); // Error!
x = x + 2; // x is 5
var y = x; // y is 5

The exceptions are the [pre/post]-[in/de]crement operations:

var x = 3;
var y = x++; // x is 4, y is 3
x = 3;
var z = ++x; // x is 4, z is 4

No Global Variables

Every top-level variable in JavaScript is global. Additionally, any variable declaration not preceded by the var statement is global. This is not the case in UnityScript, as every object-owned .js file is essentially wrapped in a class JSFileName extends MonoBehaviour {...} block. The absent-var behavior is avoided by the language prohibiting missing var keywords.

By extension, there is no global object, so the this keyword will always point to the innermost object's instance, in contrast to JS's this keyword, which will sometimes point to the global object.

Dynamic typing is inefficient

This code is valid in both UnityScript and JavaScript:

var x;
x = 3;

However, it is inefficient in UnityScript because it causes x to be dynamically typed. For faster runtime execution, use one of the two static typing syntaxes.

var x = 3; // type `int` is inferred, typed statically
var x : int; // typed statically
x = 3;


In JavaScript, privacy is rather unconventional.

function Person() { // (this is the *JavaScript* way of doing privacy)
   var secret = "I am a mass murderer."; // private
   this.speak = function() { print("Don't make me tell you my secret! "+secret); }; // prints secret
var bob = new Person();
print(bob.secret); // undefined
bob.speak(); // prints "Don't make me tell you my secret! I am a mass murderer."

In UnityScript, it can be more intuitive.

class Person { // UnityScript only; impossible in JavaScript
   private var secret : String;
   function Person() {
      secret = "I am a mass murderer.";
   function speak() {
      print("Don't make me tell you my secret! "+secret);
var bob = new Person();
print(bob.secret); // undefined
bob.speak(); // prints "Don't make me tell you my secret! I am a mass murderer."

No Bling

Dollar signs ($) are not allowed in UnityScript identifiers as they are in JS identifiers. (In JS, the symbol is often used for c-style namespacing or as the name of a do-everything function.)

var lib$cosine = 3; // ERROR! in UnityScript

No with statement

There is no with statement in UnityScript. This is probably for the best, as JavaScript's with statement causes the whole language to be slower, regardless of whether the statement is used or not. It is also considered harmful.

No Regular Expression Literals (RegExp or regex)

In JavaScript (and even JScript.NET, the language on which UnityScript is based), one can directly define a regular expression using a syntax like the following:

var sentence = /[A-Z].*[\.\?!]/; // JavaScript/JScript.NET only

UnityScript does not support this, understandably, as regular expressions are pretty uncommon in game code, and the syntax is hard to tokenize/lex.

The this keyword

In JavaScript, this can refer to one of two things, depending on the context:

  • The global object (best not explained here)
  • The object to which the current method or constructor belongs

In UnityScript, as in Oracle/Formerly Sun Java (not JavaScript), only the second of these is true. That is, this will always refer to an instance of an object, specifically the object on which a method is being called.

class Person {
   var species : String;
   var eyeColor : String;
   var hairColor : String;
   function Person(eyeColor : String) {
      species = "homo sapiens";
      this.eyeColor = eyeColor;
      hairColor = "brown";

Note that without the this. qualifier, by default, an identifier (e.g. hairColor) will refer to a class field (this.hairColor) UNLESS a variable of the same name has already been defined (e.g. the argument eyeColor). In such a case, this. must be explicitly included to disambiguate between the variable name and the field (property) name, as seen on line 7 of the code snippet.

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