Overview of differences

The p5.js language looks very similar to the Processing language with a few changes:

Because you can think of your sketch as more than just the drawing canvas, size() has been replaced with createCanvas(), to suggest the possibility of creating other elements.
frameRate(num) sets the frame rate, but the frameRate variable has been removed. To get the current frame rate, call frameRate() with no arguments.
JavaScript doesn't always load things synchronously, so there are a couple options to deal with this:
- All load methods take an optional callback argument. That is, a function that gets called after the file has been loaded.
- Alternatively, you can place load calls in a preload() method that happens before setup(). If a preload method exists, setup waits until everything inside is loaded, see this image example.
The variable mousePressed has been replaced with mouseIsPressed.
In addition to mouse events, there are touch events. The mapping is like this:
- mouseX ~ touchX
- mouseY ~ touchY
- mousePressed() ~ touchStarted()
- mouseDragged() ~ touchMoved()
- mouseReleased() ~ touchEnded()
- There is a touches[] array that contains a series of objects with x and y properties corresponding to the position of each finger.
push/popMatrix(), and push/popStyle() have been replaced with push() and pop(), the equivalent of calling both matrix and style methods together.
By default, everything is in the global namespace, and you can create your sketches like you do with Processing. However, there is something we call "instance mode" for creating a p5 sketch that plays nice with the rest of the code running on your page. See this instance mode example and this global vs instance mode tutorial.
In global mode, p5 variable and function names are not available outside setup(), draw(), mousePressed(), etc. (Except in the case where they are placed inside functions that are called by one of these methods.) What this means is that when declaring variables before setup(), you will need to assign them values inside setup() if you wish to use p5 functions. For example:
```
var n;
function setup() {
  createCanvas(100, 100);
  n = random(100);
}
```
The function println() is not available in p5.js. Use print() or console.log().
The origin (0, 0, 0) for WEBGL mode is in the center of the canvas, rather than the top left as it is in 2D mode.
WebGL uses column-major matrices. When translating 3D sketches that apply matrices from Processing to p5, matrices can be transposed to achieve the same results.
Not everything in Processing is implemented in p5.js, but we are working on it! Right now there is no PShape equivalent. The camera model in p5js is yet very basic, with only eye position and no "look at" or axis direction. See the reference for up-to-date documentation of what functions work.

Some things about JavaScript

Variables do not have a type. Use let (or the older syntax var) instead of float, int, double, long, char, String, Array, etc. You do not need to specify return types or parameter types for functions.
A var can be anything – any of the types mentioned, but also functions.
Arrays are constructed very simply (no need for Processing’s ArrayList anymore) and have many built-in features. See this array example and learn more about JS arrays here.
JavaScript uses something called prototypes to form something similar to Java class objects. See this objects example and learn more about JS objects here.

Conversion examples

Basic sketch

This is the basic setup for a Processing and p5.js sketch. Note that p5.js will also require an empty HTML file that links to the p5.js library and your sketch file in the header (see getting started).

void setup() {
  // setup stuff
}
void draw() {
  // draw stuff
}

function setup() {
  // setup stuff
}
function draw() {
  // draw stuff
}

Converting a Processing sketch to p5.js

Here are two examples of sketches that have been converted from Processing to p5.js. The changes made are shown in the comments, all the other lines remained the same.

/**
 * This example can be found in the Processing examples package
 * that comes with the Processing PDE.v
 * Processing > Examples > Basics > Form > Bezier
 * Adapted by Evelyn Eastmond
 */
function setup() {           // **change** void setup() to function setup()
  createCanvas(640, 360);    // **change** size() to createCanvas()
  stroke(255);               // stroke() is the same
  noFill();                  // noFill() is the same
}
function draw() {                         // **change** void draw() to function draw()
  background(0);                          // background() is the same
  for (var i = 0; i < 200; i += 20) {     // **change** int i to var i
    bezier(mouseX-(i/2.0), 40+i, 410, 20, 440, 300, 240-(i/16.0), 300+(i/8.0)); // bezier() is the same
  }
}

/**
 * This example can be found in the Processing examples package
 * that comes with the Processing PDE.
 * Processing > Examples > Topics > Interaction > Follow3
 * Adapted by Evelyn Eastmond
 */
var x = new Array(20);  // **change** float[] x = new float[20] to new Array(20)
var y = new Array(20);  // **change** float[] y = new float[20] to new Array(20)
var segLength = 18;                                 // **change** float to var
function setup() {                          // **change** void setup() to function setup()
  createCanvas(640, 360);                   // **change** size() to createCanvas()
  strokeWeight(9);                          // strokeWeight() is the same
  stroke(255, 100);                         // stroke() is the same
  for(var i=0; i<x.length; i++) {         // initialize the array
    x[i]=0;
    y[i]=0;
  }
}
function draw() {                           // **change** void draw() to function draw()
  background(0);                            // background() is the same
  drawSegment(0, mouseX, mouseY);           // functions calls, mouseX and mouseY are the same
  for(var i=0; i<x.length-1; i++) {         // **change** int i to var i
    drawSegment(i+1, x[i], y[i]);           // function calls are the same
  }
}
function drawSegment(i, xin, yin) {         // **change** void drawSegment() to function drawSegment(), remove type declarations
  var dx = xin - x[i];                      // **change** float to var
  var dy = yin - y[i];                      // **change** float to var
  var angle = atan2(dy, dx);                // **change** float to var, atan2() is the same
  x[i] = xin - cos(angle) * segLength;      // cos() is the same
  y[i] = yin - sin(angle) * segLength;      // sin() is the same
  segment(x[i], y[i], angle);               // function calls are the same
}
function segment(x, y, a) {                 // **change** void segment() to function segment(), remove type declarations
  push();                            		// pushMatrix() becomes push()
  translate(x, y);                          // translate() is the same
  rotate(a);                                // rotate() is the same
  line(0, 0, segLength, 0);                 // line() is the same
  pop();                              		// popMatrix() becomes pop()
}

Converting a p5.js sketch to Processing

Here are two examples of sketches that have been converted from p5.js to Processing. The changes made are shown in the comments, all the other lines remained the same.

/**
 * This example can be found in the Processing examples package
 * that comes with the Processing PDE.
 * Processing > Examples > Basics > Form > Bezier
 */
void setup() {                         // **change** function setup() to void setup()
  size(640, 360);                      // **change** createCanvas() to size()
  stroke(255);
  noFill();
}
void draw() {                          // **change** function draw() to void draw()
  background(0);
  for (int i = 0; i < 200; i += 20) {  // **change** var i to int i
    bezier(mouseX-(i/2.0), 40+i, 410, 20, 440, 300, 240-(i/16.0), 300+(i/8.0));
  }
}

/**
 * This example can be found in the Processing examples package
 * that comes with the Processing PDE.
 * Processing > Examples > Topics > Interaction > Follow3
 * Based on code from Keith Peters. 
 */
float[] x = new float[20];                       // **change** array of 0's to be float[] x = new float[20]
float[] y = new float[20];                       // **change** array of 0's to be float[] x = new float[20]
float segLength = 18;                            // **change** var to float
void setup() {                                   // **change** function setup() to void setup() 
  size(640, 360);                                // **change** createCanvas() to size()
  strokeWeight(9);
  stroke(255, 100);
}
void draw() {                                    // **change** function draw() void draw()
  background(0);
  dragSegment(0, mouseX, mouseY);
  for(int i=0; i<x.length-1; i++) {              // **change** var i to int i
    dragSegment(i+1, x[i], y[i]);
  }
}
void dragSegment(int i, float xin, float yin) {  // **change** function drawSegment() to void drawSegment(). add type declarations.
  float dx = xin - x[i];                         // **change** var to float
  float dy = yin - y[i];                         // **change** var to float
  float angle = atan2(dy, dx);                   // **change** var to float
  x[i] = xin - cos(angle) * segLength;
  y[i] = yin - sin(angle) * segLength;
  segment(x[i], y[i], angle);
}
void segment(float x, float y, float a) {        // **change** function segment() to void segment(). add type declarations.
  pushMatrix();
  translate(x, y);
  rotate(a);
  line(0, 0, segLength, 0);
  popMatrix();
}

Declaring variables

In p5.js, all variables (whether they are numbers, strings, arrays, functions, objects, whatever!) are declared using the symbol var (or in newer browsers, let or const). Unlike in Processing, you don’t have to specify the variable type.

For example, instead of:

boolean button = false;

you'd write

var button = false;

instead of:

float x = 100.3;

you'd write

var x = 100.3;

Here is a summary of the supported Processing data types (table borrowed from Getting Started with Processing).

Name	Description	Range of values
int	Integers (whole numbers)	-2,147,483,648 to 2,147,483,647
float	Floating-point values	-3.40282347E+38 to 3.40282347E+38
boolean	Logical value	true or false
char	Single character	A-z, 0-9, and symbols
String	Sequence of characters	Any letter, word, sentence, and so on
PImage	PNG, JPG, or GIF image	N/A
PFont	VLW font; use the Create Font tool to make	N/A
PShape	SVG file	N/A

What next?

Check out the p5.js reference for up-to-date documentation.
Play with the examples and demos on the tutorials page.