A brief analysis of the event delegation mechanism and deep and shallow copying in JavaScript

//Method 1: via onclick
<button onclick="clickEvent()">Click</button>

<script>
function clickEvent(){
   alert("click event");
}
</script>

//Method 2: via addEventListener
<button id="btn_button">Click</button>

<script>
var btn = document.getElementById("btn_button");
btn.addEventListener("click", function () {
  alert("click");
}, false);
</script>

[function method1() {
  console.log("method1");
}
function method2() {
  console.log("method2");
}
function method3() {
  console.log("method3");
}
var btn = document.getElementById("btn_button");
btn.addEventListener("click", method1, false); //The third parameter is the default value, which is bubbling by default. If it is set to true, it is captured. btn.addEventListener("click", method2, false);
btn.addEventListener("click", method3, false);
//Finally it will be executed in order: method1 -> method2 -> method3 
btn.removeEventListener("click", method1, false); // used to remove events btn.onclick = method1;
btn.onclick = method2;
btn.onclick = method3;
//Finally only method3 will be executed
btn.onclick = null; // used to remove the event

<ul id="container" style="display: inline-block;">
  <li id="title1">123456</li>
  <li id="title2">qwert</li>
  <li id="title3">
    <ul>
      <li id="title3inner">Text inside title3</li>
    </ul>
  </li>
</ul>

<script>
  var container = document.getElementById("container");
  var title1 = document.getElementById("title1");
  var title2 = document.getElementById("title2");
  var title3 = document.getElementById("title3");
  var title3inner = document.getElementById("title3inner");

  container.onclick = function (e) {
    alert("container");
  }
  title1.onclick = function (e) {
    alert("title1");
  }
  title2.onclick = function (e) {
    alert("title2");
  }
  title3.onclick = function (e) {
    alert("title3");
  }
  title3inner.onclick = function (e) {
    alert("title3inner");
  }
</script>

<ul id="container" style="display: inline-block;">
  <li id="title1">123456</li>
  <li id="title2">qwert</li>
  <li id="title3">QWE123</li>
</ul>

<script>
var container = document.getElementById("container");
container.onclick = function (e) {
  //console.log(e);
  if (e.target.id = "title1") {
    alert(e.target.innerText);
  }
  else if (e.target.id = "title2") {
    alert(e.target.innerText);
  }
  else if (e.target.id = "title3") {
    alert(e.target.innerText);
  }
}
</script>

let a = 3;
let b = a;
a = 4;
console.log(a); //4
console.log(b); //3
console.log(a === b); //This is a deep copy, a and b are completely different variables, each storing a value.

let arr = [1, 2, 3, 4, 5];
let brr = arr;
arr[1] = 8;
console.log(arr[1]); //8
console.log(brr[1]); //8 Why? This is a shallow copy. The array type is a reference type. The arr and brr variables only store reference addresses. They point to the array [1,2,3,4,5] together. console.log(arr === brr); //true

let arr = [1, 2, 3, 4, 5];
let brr = JSON.parse(JSON.stringify(arr));
arr[1] = 8;
console.log(brr[1]); //2, this implements the so-called deep copy