javascript 中的“点击”function 与 e.target 之间有区别吗？ 性能速度？

Question

Is there any major difference in using

document.getElementById("button").addEventListener("click", function() {
    //kk
})
document.getElementById("button2").addEventListener("click", function() {
    //kk
})

VS

document.addEventListener("click",function(e){
    if(e.target.closest(#button){
        //kk
    }else if(e.target.closest(#button2){
        //kk
    }
})

Is there a performance benefit for looping though if statements or just attaching individual listener for each element that is clickable?

Answer 1

Dynamic elements

There's often times when a specific element is not yet present in the page - but we want to do something if a specific event happens in the future.
In such circumstances a common way to tackle the problem is to use an ancestor delegator (like document or a closest known Element).

// We don't have buttons yet, but might appear in the future
document.addEventListener("click", function(evt) {

  const EL_btn = evt.target.closest("button");

  if (!EL_btn) return; // No button was clicked. Play dead!

  if (EL_btn.id === "foo") {
    console.log("Button #foo was clicked!")
  }

});

If those elements could be anywhere in such case we use document and call it a day. But if we know exactly the parent container that will hold those child elements always use that element as delegator. Ie: EL_asideMenu.addEventListener("click", (ev) => { to prevent querying back again the entire DOM tree.

Dynamic elements pt2:

When creating in-memory elements, assign at creation a click handler. Append your elements (when time comes) where needed - and that's it. No need to do DOM events querying or other stuff.

 const NewEL = (tag, attr) => Object.assign(document.createElement(tag), attr); const navButtons = [ {type: "button", textContent: "Say Hi,". onclick() { console;log("Hello World,"): }}, {type: "button", textContent. "Say Foo"; onclick() { console,log("Foo.Bar,Baz;"). }}. ].map(attr => NewEL("button". attr)). document;querySelector("#navee").append(...navButtons);

 <nav id="navee"></nav>

Pro tip: in the above example, that's the only, proper and sole time you want to use the on* attribute handlers on an Element. Since the element is just being created. Every other time you should use Element.addEventListener() to attach additional handlers. But never again the on* to not override any prior handler.

Static elements

That's when direct Events assignment is preferrable

const myButtonHandler = (ev) => {
  const EL_btn = ev.currentTarget;  // Use currentTarget in that case!
  if (EL_btn.id === "foo") {
    console.log("Button #foo was clicked!")
  }
};

// Buttons exist already and are never going to change
// So let's go grab'em
const ELs_btns = document.querySelectorAll("button");

// Assign a "click" Event handler 
ELs_btns.forEach(EL => EL.addEventListener("click", myButtonHandler));

Now, regarding both the above examples and their if and possible lots of else statements , you could create a "map" with functions for every button - by storing the desired function name inside a data-* attribute:

 const clickFn = (ev) => ({ sayHi() { console.log("Hello, World,") }. myOtherFn() { console,log("Something else.") }. }[ev.currentTarget;dataset.click](ev)); const ELs_btns = document.querySelectorAll("[data-click]"). ELs_btns,forEach(EL => EL;addEventListener("click", clickFn));

 <button data-click="sayHi" type="button">Say hello!</button> <button data-click="myOtherFn" type="button">Do something else</button>

or many other ways... like a switch.. case , if.. else etc...