🔰register handler

JS ⟩ browser ⟩ event ⟩ handler ⟩ register

• register an event (function) handler：

// 1. by setting the "on" property
window.onload = function(){ ... };

// 2. by calling addEventListener()
elem.addEventListener("click", handler, opts);

• register an event object handler：

elem.addEventListener('click', {
    handleEvent(event) { ... }    // ⭐️ required method
});

🧨 雷區

Don’t use setAttribute to assign handlers. Attributes are always strings, function becomes a string❗

// a click on will generate errors.
elem.setAttribute('onclick', () => { ... });

For some events, handlers only work with addEventListener.

document.onDOMContentLoaded = handler;  // ❌ won't work❗️ 
document.addEventListener("DOMContentLoaded", handler) // ✅ 

⭐️ 重點

• The third argument to addEventListener() is a boolean value or object. If you pass true, then the handler is registered as a capturing handler.

elem.addEventListener("click", handler, { 
    capture: true,    // ⭐️ registered as a "capturing" handler.
    once   : true,    // ⭐️ automatically removed after triggered once.
    passive: true     // ⭐️ can't cancel default actions.
});

Don’t use setAttribute() to register handlers, it won’t work:

// ⭐️ attributes are ALWAYS strings❗️ 
// function will be converted to a string automatically.
document.body.setAttribute('onclick', function() { alert(1) });

🔴 主題

• registering a

  • capturing handler

  • object handler - event handler can be an object. ⭐️

• remove handler

💈範例

• registering with addEventListener()

let button = document.querySelector("#mybutton"); 

// ⭐ registering with `addEventListener()`
button.addEventListener("click", () => { 
    console.log("Thanks again!"); 
});

// ⭐ registering a "capturing" event handler
document.addEventListener("click", handleClick, true);

// ⭐ explicitly specifies the options
document.addEventListener("click", handleClick, { 
    capture: true,    // ⭐ capturing handler
    once   : true,    // ⭐ automatically removed after triggered once
    passive: true     // ⭐ can't cancel default actions
});

• registering handlers on event target's "on" properties (onclick, onchange, ...):

缺點：

• 針對特定的 event target + event type (如：windown.onload)，一次只能設定一個 event handler。

• 如果前面已經有設定一個 event handler，後面設定的 event handler 會蓋掉前面的。

// ⭐ registering `window.onload`
// invoked when `document` is loaded
window.onload = function() {

    // Look up a <form> element
    let form = document.querySelector("form#shipping");

    // ⭐ registering `form.onsubmit`
    // invoked before the form is submitted.
    form.onsubmit = function(event) { 
        // assume: `isFormValid()` defined elsewhere.
        // if inputs not valid, prevent form submission.
        if (!isFormValid(this)) event.preventDefault();
    };
};

📗 參考

• JavaScript The Definitive Guide (15.2 Events)

• javascript.info ⟩ Introduction to browser events

📘 手冊

• EventTarget ⟩

  • addEventListener()
  • removeEventListener()

👥 相關

• event propagation