1. Compatiblity.
2. addEventListener, attachEvent.
3. stopPropagation, cancelBubble.
4. onclick -> DOM0,
5. onload, onbeforeunload, onresize, onscroll, focus.
6. mouse-event: click, dblclick, mousedown, mouseup, mousemove, mouseenter, mouseleave, mouseover;
Properties: ClientX/Y, pageX/Y, screenX/Y,
7. keyboard: keydown, keypress, keyup.
8. Packaging for Compatibility issues.
.find(u => u.id == id);
.findIndex(u => u.id == id);
const fs = require("fs");
const path = require("path");
let filedata = fs.readFileSync(path.join(__dirname, '15_Iterator.js'), 'utf8');
Object.assign(obj1, obj2); // deep-copy obj