let

Declares a new variable.

A variable is a container for a value. For example, a variable might contain a creature's x-coordinate as a Number or its name as a String. Variables can change value by reassigning them as follows:

// Declare the variable x and assign it the value 10.
let x = 10;

// Reassign x to 50.
x = 50;

Variables have block scope. When a variable is declared between curly braces {}, it only exists within the block defined by those braces. For example, the following code would throw a ReferenceError because x is declared within the setup() function's block:

function setup() {
  createCanvas(100, 100);

  let x = 50;
}

function draw() {
  background(200);

  // x was declared in setup(), so it can't be referenced here.
  circle(x, 50, 20);
}

Variables declared outside of all curly braces {} are in the global scope. A variable that's in the global scope can be used and changed anywhere in a sketch:

let x = 50;

function setup() {
  createCanvas(100, 100);
}

function draw() {
  background(200);

  // Change the value of x.
  x += 10;

  circle(x, 50, 20);
}