text: Q:
  
  Particle system within a drawing in p5.js
  
  I'm trying to create a particle system within a basic-shape drawing. The particles should emit from the top of the leftmost structure (commented "Heading") and flow downwards, similar to a fountain. I have a majority of the Particle class figured out, but error messages related to the variables arise when I run the sketch.
  Applied some edits based on answers, but cannot get the particle system to display. Amended code listed below.
  var particles = [ ];
  
  function Particle() {
    initialization() 
      this.x = 135;
      this.y = 75;
      this.vx = random(-1, 1);
      this.vx = random(-5, -1);
      this.alpha = 255;
    
    update() 
      this.x += this.vx;
      this.y += this.vy;
      this.alpha -= 5;
    
    show() 
      noStroke();
      fill ('#1E740C');
      ellipse (this.x, this.y, 8, 8);
  }
  
  function setup() {  
    createCanvas(600, 400);
    
    //Particle Array
    for(var i = 0; i < particles.length; i++) {
    particles[i] = new Particle();
    }
  }
  
  function draw() {  
    background('#87D9E8'); 
    stroke(0);
    strokeWeight(1);
    
      //Loop array and alter each element
    for(var i=0; i < particles.length; i++) {
      particles[i].show();
      particles[i].update();
    }
    
  //SLIME GEYSER
    
  ///Center Pipe
    fill ('#1FA600');
    rect(340, 75, 20, 290);
    rect(340, 75, 130, 20);
    
  ////Body
    fill ('#1FA600');
    ellipse (305, 495, 450, 400);
    
  ///Heading (Spout)
   //rect(100, 140, 80, 140, 15); //Top segment
   //triangle (100, 150, 180, 150, 140, 100); //"Neck" segment
   // rect(120, 75, 40, 40); //SPOUT TIP
   // ellipse(140, 115, 100, 45); //Spout "bulb"  
    rect(120, 350, 40, 30); //Bottom pipe
    rect(100, 290, 80, 60, 20); //Bottom segment
    rect(100, 277, 80, 15); //Midway segment
    
  //////Pressure Condenser Unit Grille
    fill('#989E9B');
    rect(390, 135, 140, 130);
    line (410, 305, 410, 100);
    line (430, 305, 430, 100);
    line (450, 305, 450, 100);
    line (470, 305, 470, 100);
    line (490, 305, 490, 100);
    line (510, 305, 510, 100);
    
  ////Pressure Condenser Unit
    fill ('#1FA600');
    arc(460, 265, 140, 140, 0, HALF_PI+HALF_PI);
    arc(460, 135, 140, 140, PI, 0);
    
  ////Tube
    noFill();
    stroke('#1FA250');
    strokeWeight(10);
    beginShape();
    vertex(110, 270);
    quadraticVertex(10, 200, 110, 150);
    endShape();
  
  }
  
  A:
  
  First of all you don't have to declare a global Particle variable, the class would do it.
  You aren't calling the show function on the particles array so it won't display the particles.
  Instead you can do something like this,
  function Particle() {
                                  
                  this.x = 120; 
                  this.y = 200; 
                  this.vx = random(-1,1); 
                  this.vy = random(-5,1); 
                  
                  this.alpha = 255; 
              
                  this.show = function() { 
                                  noStroke(); 
                                  fill("#1E740C"); 
                                  
                                  ellipse(this.x, this.y, 8, 8);
                                  
                  }
                  
                  this.update = function() {
                                  this.x += this.vx;
                                  this.y += this.vy;
                                  //console.log("run")
                  }
  }
  
  var p = [];
  
  function setup() {
      createCanvas(345,400);
                  
                  for(var i = 0; i < 10; i++) {
                                  p[i] = new Particle();
                  }
  }
  
  function draw() {
                  
                  background(0);
                  
                  for(var i = 0; i < p.length; i++) {
                                  
                                  p[i].show();
                                  p[i].update()
                  }
  }
  
  That's probably the easiest way to create the particles system. If this answer wasn't helpful then checkout these links:
  CodingTrain: https://thecodingtrain.com/CodingChallenges/078-simple-particle-system.html
  Or check out p5's own example:
  https://p5js.org/examples/simulate-multiple-particle-systems.html
  
  