How do I make this piece of code work?

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Javascript

I am trying to make the particles explode at 250,250 when b==250 but its not working

What I have tried:

var a=500;
var b=490;
var pA;


function setup() {
var canvas=createCanvas(500,500);
canvas.parent("easel");
textSize(50);
angleMode(DEGREES);

pA=new Array(500);
for(var i=0;i<500;i++){
pA[i]=new Particle(250,250,0,5,-5,5);

}


}


function draw() {
background(255,255,0);




stroke(255,0,0);
strokeWeight(3);
line(250,a,250,b);

if(a>250){
a--
};
if(b>250){
b--
};
if(b==250) {
stroke(255,0,0);
strokeWeight(2);
for(var i=0;i<500;i++){
point(pA[i].x, pA[i].y);
pA[i].x+=pA[i].vx;
pA[i].y+=pA[i].vy;
}
};

function Particle(x,y,vx1,vx2,vy1,vy2){
this.x=x;
this.y=y;
this.vx=random(vx1,vx2);
this.vy=random(vy1,vy2);
}

\
}