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JASS (Just Another Scripting Syntax) is an event driven scripting language used in Blizzard Entertainment's Warcraft III game. Map creators can use it in the World Editor to create triggers and AI (artificial intelligence) scripts.



The language provides an extensive API that gives programmers control over nearly every aspect of the game world. It can, for example, execute simple GUI functions such as, giving orders to units, change the weather and time of day, play sounds and display text to the player, and manipulate the terrain. JASS can also create powerful functions such as trackables, which detects if a mouse goes over or hits a position, GetLocalPlayer(), which can cause disconnects if used improperly (such as using handles with GetLocalPlayer(). It has a syntax similar to Turing and Delphi, although, unlike those languages, it is case sensitive. JASS primarily uses procedural programming concepts, though popular user-made mods to Blizzard's World Editor program have since added C++-like object-oriented programming features to the syntax of JASS.

Sample code

The following function creates a string containing the message "Hello, world!" and displays it to all players:

function Trig_JASS_test_Actions takes nothing returns nothing
 call DisplayTextToPlayer(GetLocalPlayer(), 0, 0, "Hello, world!")

or if you want this only for one player:

function Trig_JASS_test_Actions takes player p returns nothing
 call DisplayTextToPlayer(p, 0,0, "Hello, world!")

And if you want to print the message 90 times and show the iterator:

function Trig_JASS_test_Actions takes player p returns nothing
 local integer i=0
   exitwhen i==90
   call DisplayTextToPlayer(p, 0,0, "Hello, world! "+I2S(i))
   set i=i+1

Basic syntax

Syntax of JASS is similar to Turing. It is context free. Examples of basic syntax are shown below:

function syntax_Example_Sum takes integer i, real r returns real //function declaration must include: the keyword "function", 
                                                                 //the function name, parameters (if any) and return type (if 
                                                                 //it returns something)
  return i + r //return statements must begin with the keyword "return"
endfunction //the keyword "endfunction" signals the end of a function block

function syntax_Example takes nothing returns nothing 
  local integer i //declaring a local variable requires the modifier "local", the variable's data type, and the variable name
  local real r = 5.0 //local variable declarations must come before anything else in a function and variables may be 
                     //initialized on declaration

  //separated statements MUST be placed on separate lines

  set i = 6 //the keyword "set" is used to rebind variables
  call syntax_Example_Sum( i, r ) //function calls must be preceded by the keyword "call"
  set r = syntax_Example_Sum( i, r ) //the "call" keyword is omitted when accessing a function's return value


JASS is statically-typed, and its types can be separated into two classes: natives and handles. The native types are:

  • integer (32-bit signed)
  • real (32-bit floating point numbers, similar to the float type in Java)
  • string (limit is 1023 signs, actually to provide save/load compatibility it's required to use only 1013 signs)
  • boolean
  • code

All other types are considered non-native. The native types behave very similarly to primitive types in other programming languages. Handle types, however, behave more like objects. Handle types often represent an "object" within the game (units, players, special effects, etc.). Similarly to how Java treats Objects, all variables and parameters in JASS of handle types are treated as values, but in reality those values are nothing but references to the handle objects. This becomes important when dealing with garbage collection because handles, if not properly cleaned up, can cause significant performance issues. Additionally, local variables do not properly dereference handles when they go out of scope. If they are not nullified properly, handle indices will not be garbage collected and will eventually leak. Also, any references to handles themselves take up some memory space. Users may experience reduced performance if they are not nullified, though on a much smaller scale.

function garbage_Collection_Example takes string effectPath, real x, real y returns nothing
 local effect specialEffect = AddSpecialEffect( effectPath, x, y ) //uncleaned handle types will continue to take up system resources

function garbage_Collection_Example2 takes string effectPath, real x, real y returns nothing
 local effect specialEffect = AddSpecialEffect( effectPath, x, y )
 set specialEffect = null //setting the variable to null is not enough, since it is only a reference to the handle; 
                          //the handle still exists

function garbage_Collection_Example3 takes string effectPath, real x, real y returns nothing
 local effect specialEffect = AddSpecialEffect( effectPath, x, y )
 call DestroyEffect( specialEffect ) //we destroy (clean up) the handle to free up memory
 set specialEffect = null 

function garbage_Collection_Example4 takes effect e returns nothing
  //do stuff
  call DestroyEffect( e )
  //parameters do not have to be nullified

Another property of handle types worth noting is that all handle types are treated as if they were children of the "handle" type. Some of these children types have their own children types, and so on. Handle variables may reference its own specific handle type or any children type. For example:

function Trig_JASS_handle_Example_Child takes widget w, widget w2 returns nothing
  //do stuff

function handle_Example takes real x, real y returns nothing
  local widget w //widget is a handle type with children type unit and destructible
  local unit u = CreateUnit( 'hfoo', x, y )
  local destructible d = CreateDestructible( 'ATtr', x, y )
  set w = u //acceptable
  call Trig_JASS_handle_Example_Child( w, d ) //acceptable

Type casting

Of the primitive types, type casting between integer, real, and string is officially supported by the language. JASS supports both implicit and explicit type casting.

Implicit casting only occurs from real to integer. For example:

function type_Casting_Example takes nothing returns real
  local integer i = 4
  local real r = 5 //implicit cast of 5 to real to satisfy variable type
  if ( i < r ) then //implicit cast of i to real
    set r = r / i //implicit cast of i to real in order to carry out real division
  return i //XXX: NOT allowed; JASS does not allow implicit casting from integer to real to satisfy return types

The JASS library provides several functions for explicit type casting:

  • I2R(): casts integer to real
  • R2I(): casts real to integer
  • I2S(): casts integer to string
  • R2S(): casts real to string

An important property of handle types related to type casting is that since all variables of handles are just references, they can all be treated (and are treated) as integers. Each instance of a handle is assigned a unique integer value that essentially acts as an identifier for the handle. Therefore, type casting from handles to integers, although technically not supported by JASS, is possible in practice because implicit casting from handle types to integer can and will occur if the code is written in a certain way, for example:

function H2I takes handle h returns integer
  return h

If the game ever reached the line "return h", it would in fact actually cast the handle to an integer and return the value. However, Blizzard never intended for JASS to be used this way, and so the JASS compiler will actually throw an error, warning the programmer that the function isn't returning the correct type. However, JASS programmers have found and exploited a now famous bug in the JASS compiler's syntax checker: the so-called "return bug". Essentially, the compiler will only make sure that the last return statement in a function returns the correct type. Therefore, the following code compiles without error and can be used to cast handles to integers

function H2I takes handle h returns integer
  return h //the function will stop executing after the first return statement, i.e. this one
  return 0 //the compiler will only check this statement for syntax accuracy, but it is in reality unreachable code

This bug has been fixed in patch 1.23b, although it was not completely fixed until patch 1.24b. Users have to use new hashtable natives instead of their return bug counterparts. While this bug has been fixed in patch 1.24b, another return bug has been discovered by users, known as the Return Nothing bug. The Return Nothing bug has was then fixed by Blizzard in patch 1.23c.

The return nothing bug lets the user get the last value returned by any function, even as another type. To properly utilize the bug, the desired return must be made in a separate function and a return in the calling function should be made impossible.

function ReturnHandle takes handle h returns handle
   return h
function H2I takes handle h returns integer
   call ReturnHandle(h) //This sets the last returned value to 'h'.
   if false then
       return 0 //This can never occur, so the game uses the last returned value as this function's returned value instead.
       //It will even return the last returned value as a different type, in this case an integer.


JASS supports one-dimensional arrays of any type (excluding code). The syntax to declare arrays and access members in an array is outlined in the code below.

function array_Example takes nothing returns nothing
  //arrays cannot be initialized at declaration and their members will hold arbitrary values
  local integer array numbers
  local unit array units
  local boolean array booleans
  local integer i = 1
  set numbers[0] = 1 //the syntax to initialize an array member is identical to that for any other variable, only a set of 
                     //brackets: [] must immediately follow the variable name with the index value of the specific member
                     //to initialize inside the brackets
  set units[0] = CreateUnit( 'hfoo', 0, 0 ) //indexes for arrays always start at 0
    exitwhen ( i > 8191 ) //the maximum size for arrays in JASS is 8192, which means that the last index of any array can 
                          //only be 8191,  to provide save/load compatibility it's required to use maximum index 8190
    set numbers[i] = i    //variables can substitute for constants when specifying the index value in the brackets
    set units[numbers[i]] = null //array members can also substitute
    set booleans[i-1] = true //arithmetic (or functional) operations are also acceptable

  if ( booleans[200] == true ) then //to access an array member, again the syntax is the same as for normal variables, only 
                                   //with the addition of the brackets with the index value inside
    set numbers[200] = -200 //you can re-assign members of the array to different values

One limitation of arrays in JASS is that they cannot be returned by functions or passed as parameters to other functions, though array members may be returned (in a function that returns a unit, u[0] may be returned if u is an array of type unit).

Patch 1.24 and return bug

Blizzard Entertaiment has decided to deprecate any use of return bugs with Warcraft III patch 1.24. This is done as an attempt to fix certain security vulnerabilities. Once the patch is deployed, any map that contains the script code with returns bugs will not run properly. All maps using return bugs must use newly available hashtable natives to be compatible with newer versions.

The patch is currently applied on all Battle.net servers.


vJass is a set of user-made extensions to JASS. It introduces object-oriented programming features to the language, such as structs, encapsulation, and polymorphism.[1] Strictly speaking, vJass does not add anything to the JASS library but instead mostly uses JASS arrays and integers used as their indices. The extension relies on a custom-made compiler that compiles vJass code to strict JASS code. In this manner, no additional mods for either the World Editor program or Warcraft III are required, and maps made using vJass code are fully compatible with any copy of the game, even those without the compiler. The most convenient Tool for Map-Developers which want to use vJass is currently the JassNewGen-Pack, which includes several enhancements for the Warcraft III-World-Editor (including a vJass precompiler). JassNewGen Pack is only fully compatible to Warcraft III 1.21b, however most of its functions work for 1.22 as well.

Known Issues

The JASS interpreter of Warcraft III up to version 1.23 doesn't check memory region boundaries. This allows execution of arbitrary bytecode through a map, meaning that practically anything, including malware (virus, trojans, etc.), can be engineered into a map to be executed and infect your computer. Blizzard Entertainement is aware of this issue and applied a temporary workaround to games hosted on Battle.net. They are also preparing a permanent fix for LAN and singleplayer games.[2] This issue was addressed with the release of version 1.24.[3]


  1. vJass Documentation
  2. Battle.net Forums - Warcraft III Custom Map Security Warning
  3. Dota Allstars Blog - Warcraft III version 1.24 changelog

External links




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