keyPressed vs. keyIsPressed - Tutorial

mySketch
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let a;
​
function setup() {
  createCanvas(400, 400);
  a = 100;
}
​
function draw() {
  background(255);
  
  // Add some text behind the shape so that we can 
  // more easily visualize the transparency
  stroke(0);
  textSize(32);
  text("hi", width/2 - 12, height/2 + 12);
  
  // Draw that shape!
  noStroke();
  fill(169, 255, 231, a);
  ellipse(width/2, height/2, 200);
  
  // Update the alpha based on the key I'm pressing
  if (keyIsPressed) {
    if (keyCode == LEFT_ARROW) {
      // Lower the alpha if it can be lowered
      if (a > 0) {
        a -= 1;
      }
    } else if (keyCode == RIGHT_ARROW) {
      // Raise the alpha if it can be raised
      if (a < 255) {
        a += 1;
      }
    }
  }
}