Snake game in Games with javascript 

I am not able to understand withinBoundary () function in Snake game. Can you please explain it?

Code:

var numberofsegments = 20;

var segmentSize = 20;

var snakeheadX = 0;

var snakeheadY = 0;

var velocityX =0;

var velocityY = 0;

var taillength = 3

var tailarray = []

function setup() {

createCanvas(400, 400);

snakeheadX = numberofsegments/2;

snakeheadY = numberofsegments/2;

frameRate(10)

}

function draw() {

background(220);

// cut extra tail

while(tailarray.length > taillength)

{

tailarray.shift()

}

// do traw tail

tailarray.push({x:snakeheadX, y:snakeheadY})

for(let i=0; i<tailarray.length; i++)

{

rect(tailarray[i].x*segmentSize,

tailarray[i].y*segmentSize,

segmentSize,

segmentSize)

}

snakeheadX = snakeheadX+velocityX

snakeheadY = snakeheadY+velocityY

withinBoundary();

rect(snakeheadX*segmentSize,snakeheadY*segmentSize,segmentSize,segmentSize);

}

function keyPressed()

{

if(keyCode == UP_ARROW)

{

velocityX = 0;

velocityY = -1;

}

if(keyCode == DOWN_ARROW)

{

velocityX = 0;

velocityY = 1;

}

if(keyCode == RIGHT_ARROW)

{

velocityX = 1;

velocityY = 0;

}

if(keyCode == LEFT_ARROW)

{

velocityX = -1;

velocityY = 0;

}

}

function withinBoundary()

{

if(snakeheadX < 0)

{

snakeheadX = numberofsegments - 1

}

if(snakeheadX > numberofsegments - 1)

{

snakeheadX = 0

}

if(snakeheadY < 0)

{

snakeheadY = numberofsegments - 1

}

if(snakeheadY > numberofsegments - 1)

{

snakeheadY = 0

}

}