HISE Javascript Tutorial

To make HISE Scripting perform well in a Audio-DSP context, a few major adaptations had to be made to it's JavaScript-Engine Implementation. It does'nt include the latest [ES6] specifications and had also to be stripped of a few known JS paradigms and is therefore not fully standard compliant. Take a look at HISE Additions for the main differences between a standard JavaScript and the HiseScript implementation.

This Tutorial builds upon the A re-introduction to JavaScript Tutorial, reduced to the scope of Scripting in HISE .


JavaScript is an object oriented dynamic language with types and operators, standard built-in objects, and methods. Its syntax is derived from the Java and C languages so many structures from those languages apply to JavaScript as well.

One of the key differences is that JavaScript does not have classes. Instead, the class functionality is accomplished by object prototypes. The other main difference is that functions are objects, giving functions the capacity to hold executable code and be passed around like any other object.

Data Types

Let's start off by looking at the building block of any language: the types. JavaScript programs manipulate values, and those values all belong to a type. JavaScript's types are:

Arrays and Functions are special kinds of Objects. ... oh, and undefined and null , which are... slightly odd.


Numbers in JavaScript are "double-precision 64-bit format IEEE 754 values", according to the spec. This has some interesting consequences. There's no such thing as an integer in JavaScript, so you have to be a little careful with your arithmetic if you're used to math in C or Java. Watch out for stuff like:

0.1 + 0.2 == 0.30000000000000004

In practice, integer values are treated as 32-bit ints (and are stored that way in some browser implementations), which can be important for bit-wise operations.

The standard arithmetic operators are supported, including addition, subtraction, modulus (or remainder) arithmetic and so forth. There's also a built-in object that I forgot to mention earlier called Math if you want to perform more advanced mathematical functions and constants:

var r = Math.sin(3.5);
var circumference = Math.PI * (r + r);

You can convert a string to an integer using the built-in parseInt() function. This takes the base for the conversion as an optional second argument, which you should always provide:

parseInt("123", 10); // 123
parseInt("010", 10); // 10

If you don't provide the base, you can get surprising results:

parseInt("010"); // 8

That happened because the parseInt() function decided to treat the string as octal due to the leading 0.

JavaScript has the special values Infinity and -Infinity:

1 / 0; //  Infinity
-1 / 0; // -Infinity

The parseInt() and parseFloat() functions parse a string until they reach a character that isn't valid for the specified number format, then return the number parsed up to that point. However the "+" operator simply converts the string to NaN if there is any invalid character in it. Just try parsing the string "10.2abc" with each method by yourself in the console and you'll understand the differences better.


Strings in JavaScript are sequences of characters. More accurately, they are sequences of Unicode characters, with each character represented by a 16-bit number.


To find the length of a string, access its length property:

"hello".length; // 5

There's our first brush with JavaScript objects! Did I mention that you can use strings like objects too? They have methods as well that allow you to manipulate the string and access information about the string:

"hello".charAt(0); // "h"
"hello, world".replace("hello", "goodbye"); // "goodbye, world"
"hello".toUpperCase(); // "HELLO"

String API


JavaScript has a boolean type, with two possible values true and false (both of which are keywords). Any value can be converted to a boolean according to the following rules:

  1. false , 0 , the empty string ("") , NaN , null , and undefined all become false .
  2. all other values become true .

However, conversion is rarely necessary, as JavaScript will silently perform this conversion when it expects a boolean, such as in an if statement For this reason, we sometimes speak simply of "true values" and "false values," meaning values that become true and false, respectively, when converted to booleans. Alternatively, such values can be called "truthy" and "falsy".

Boolean operations such as && (logical and), || (logical or), and ! (logical not) are supported.

null / undefined

JavaScript distinguishes between null , which is a value that indicates a deliberate non-value (and is only accessible through the null keyword), and undefined , which is a value of type undefined that indicates an uninitialized value — that is, a value hasn't even been assigned yet. In JavaScript it is possible to declare a variable without assigning a value to it. If you do this, the variable's type is undefined. undefined is actually a constant.


New variables in JavaScript are declared using the var keyword:

var a;
var name = "simon";

HISE Advice: Because of the real-time-safe requirements of HISE the declaring of var variables is discouraged in all audio related callbacks since it will result in unpredictable performance with drop outs & stuff. Best practice is to declare all variables in the onInit() callback and assign values to the variable in the other callbacks.

HiseScript also features four other ways of declaring variables:

Please have a look at the HISE Additions Chapter to learn more about custom variables and scopes in HISE .

An important difference from other languages like Java is that in JavaScript, blocks do not have scope; only functions have scope. So if a variable is defined using var in a compound statement (for example inside an if control structure), it will be visible to the entire function.


JavaScript's numeric operators are + , - , * , / and % - which is the remainder operator. Values are assigned using = , and there are also compound assignment statements such as += and -= . These extend out to x = x y .

x += 5
x = x + 5

You can use ++ and -- to increment and decrement respectively. These can be used as prefix or postfix operators.

The + operator also does string concatenation:

"hello" + " world"; // "hello world"

If you add a string to a number (or other value) everything is converted in to a string first. This might catch you up:

"3" + 4 + 5;  // "345"
 3 + 4 + "5"; // "75"

Adding an empty string to something is a useful way of converting it.

Comparisons in JavaScript can be made using < , > , <= and >= . These work for both strings and numbers. Equality is a little less straightforward. The double-equals operator performs type coercion if you give it different types, with sometimes interesting results:

"dog" == "dog"; // true
1 == true; // true

To avoid type coercion, use the triple-equals operator:

1 === true;    // false
true === true; // true

There are also != and !== operators.

JavaScript also has bitwise operations. If you want to use them, they're there.

Control structures

JavaScript has a similar set of control structures to other languages in the C family. Conditional statements are supported by if and else ; you can chain them together if you like:

If / else

var name = "kittens";
if (name == "puppies") {
  name += "!";
} else if (name == "kittens") {
  name += "!!";
} else {
  name = "!" + name;
name == "kittens!!"

For loop

JavaScript's for loop is the same as that in C and Java: it lets you provide the control information for your loop in a single line.

for (var i = 0; i < 5; i++) 
  // Will execute 5 times

This variant with a pre-defined reg variable is a bit faster though.

reg i = 0;
for (i; i < 5; i++) 
  // Will execute 5 times

You can loop through an array with the for (i in array) syntax.

const var array = [1,2,3,4]
for (i in array) 

IMPORTANT: Go easy on the loops in the MIDI callbacks.

While loops

JavaScript has while loops and do-while loops . The first is good for basic looping; the second for loops where you wish to ensure that the body of the loop is executed at least once:

while (true) {
  // an infinite loop!

var input;
do {
  input = get_input();
} while (inputIsNotValid(input))

AND / OR operators

The && (AND) and || (OR) operators use short-circuit logic, which means whether they will execute their second operand is dependent on the first. This is useful for checking for null objects before accessing their attributes:

var name = o && o.getName();

Or for setting default values:

var name = otherName || "default";

Ternary operator

JavaScript has a ternary operator for conditional expressions:

var allowed = (age > 18) ? "yes" : "no"; 
// if (condition == true) ?(become) "yes" :(else) "no";

Switch statement

The switch statement can be used for multiple branches based on a number or string:

switch(action) {
  case 'draw':
  case 'eat':

If you don't add a break statement, execution will "fall through" to the next level. This is very rarely what you want — in fact it's worth specifically labeling deliberate fallthrough with a comment if you really meant it to aid debugging:

switch(a) {
  case 1: // fallthrough
  case 2:


JavaScript objects can be thought of as simple collections of "name-value pairs". As such, they are similar to:

The fact that this data structure is so widely used is a testament to its versatility. Since everything (bar core types) in JavaScript is an object, any JavaScript program naturally involves a great deal of hash table lookups. It's a good thing they're so fast!

The "name" part is a JavaScript string, while the value can be any JavaScript value — including more objects. This allows you to build data structures of arbitrary complexity.

There is only one way to create an a plain object. (The new operator is not supported in HISE )

var obj = {};

This is called the _object literal syntax_, and is more convenient. This syntax is also the core of the JSON format and should be preferred at all times.

Object literal syntax can be used to initialize an object in its entirety:

var obj = {
  name: "Carrot",
  "for": "Max",
  details: {
    color: "orange",
    size: 12

Attribute access can be chained together:

obj.details.color; // orange
obj["details"]["size"]; // 12


Arrays in JavaScript are actually a special type of object. They work very much like regular objects (numerical properties can naturally be accessed using [] syntax) but they have one magic property called 'length'. This is always one more than the highest index in the array.

One way of creating arrays is as follows:

var a = [];
a[0] = "dog";
a[1] = "cat";
a[2] = "hen";
a.length; // 3

HISE does not support the var a = new Array() definition style.

A more convenient notation is to use an array literal:

var a = ["dog", "cat", "hen"];
a.length; // 3

Note that array.length isn't necessarily the number of items in the array. Consider the following:

var a = ["dog", "cat", "hen"];
a[100] = "fox";
a.length; // 101

Remember — an array index starts with 0 . The length of the array is therefore one more than its highest index.

If you query a non-existent array index, you get undefined :

typeof a[90]; // undefined

If you take the above into account, you can iterate over an array using the following:

for (var i = 0; i < a.length; i++) {
  // Do something with a[i]

This is slightly inefficient as you are looking up the length property once every loop. An improvement is to declare the array-length variable up-front

const var len = a.length;
for (var i = 0, i < len; i++) {
  // Do something with a[i]

You can iterate over an array using a loop.

for (value in a) {

If you want to append an item to an array simply do it like this:


Arrays come with a number of methods. See the full API documentation for Array methods.

Array API


Along with objects, functions are the core component in understanding JavaScript. The most basic function couldn't be much simpler:

function add(x, y) {
  var total = x + y;
  return total;

This demonstrates a basic function. A JavaScript function can take 0 or more named parameters. The function body can contain as many statements as you like, and can declare its own variables which are local to that function. The return statement can be used to return a value at any time, terminating the function. If no return statement is used (or an empty return with no value), JavaScript returns undefined.

The named parameters turn out to be more like guidelines than anything else. You can call a function without passing the parameters it expects, in which case they will be set to undefined.

add(); // NaN 
// You can't perform addition on undefined

You can also pass in more arguments than the function is expecting:

add(2, 3, 4); // 5 
// added the first two; 4 was ignored

Note that JavaScript functions are themselves objects and you can add or change properties on them just like on objects we've seen in the Objects section.

Take a look at inline functions for function with scope.