const sourceText = "プログラミング難しい";

let curIndex = 0;

function setup() {

  createCanvas(700, 700);

  frameRate(6);

}

function draw() {

  background(50);

  fill(255);

  textSize(300);

  textAlign(CENTER, CENTER);

  text(

    sourceText.substring(curIndex, curIndex+1),

    width/2, height/2);

	  curIndex++;

}