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g43riko / Visible heap map

// ==UserScript==
// @name         Visible heap map
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @include *
// @copyright 2018, g43riko (https://openuserjs.org/users/g43riko)
// @license MIT
// ==/UserScript==
(function() {
    const ACTIVATION_KEY = 83; // S
    const canvas = document.createElement("canvas");
    const ctx = canvas.getContext("2d");

    canvas.width = window.innerWidth;
    canvas.height = (document.height !== undefined) ? document.height : document.body.offsetHeight;
    canvas.style.position = "absolute";
    canvas.style.top = canvas.style.left = 0;
    canvas.style.zIndex = 10000;
    canvas.style.pointerEvents = "none";
    canvas.style.display = "none";
    document.body.appendChild(canvas);

    const oldOnMouseMove = window.onmousemove;
    const oldOnKezDown = window.onkeydown;

    window.onmousemove = (e) => {
      ctx.beginPath();
      ctx.arc(e.pageX, e.pageY, 50, 0, 2 * Math.PI, false);
      ctx.fillStyle = 'rgba(0, 0, 0, 0.01)';
      ctx.fill();
      oldOnMouseMove && oldOnMouseMove(e);
    }

    window.onkeydown = (e) => {
        if (e.keyCode === ACTIVATION_KEY) {
            const oldOnKeyUp = window.onkeyup;
            canvas.style.display = "block";
            window.onkeyup = (ee) => {
                canvas.style.display = "none";
                oldOnKeyUp && oldOnKeyUp(ee);
            }
        }
        oldOnKezDown && oldOnKezDown(e);
    }
})()