mouseX()
A Number system variable that tracks the mouse's horizontal position.
mouseX keeps track of the mouse's position relative to the top-left corner of the canvas. For example, if the mouse is 50 pixels from the left edge of the canvas, then mouseX will be 50.
Examples

function setup()
size(100, 100)
describe("A vertical black line moves left and right following the mouse's x-position.")
end
function draw()
background(200)
-- Draw a vertical line that follows the mouse's x-coordinate.
line(mouseX, 0, mouseX, 100)
end

function setup()
size(100, 100)
describe("A gray square. The mouse's x- and y-coordinates are displayed as the user moves the mouse.")
end
function draw()
background(200)
fill(0)
-- Style the text.
textAlign(CENTER)
textSize(16)
-- Display the mouse's coordinates.
text('x: '..mouseX..' y:'..mouseY, 50, 50)
end
Syntax
mouseX
Returns
Number: Current x-coordinate of mouse.