Javascript無法在onclick事件中訪問函數內的全局對象

Question

我們有通常的示例代碼，一切正常。

（ myFunction在 onclick 事件上觸發。）

"use strict";
console.clear();

const obj = {
    name: "falana",
    count: 0
};

function myFunction() {
    obj.count++;
    console.log(obj.count);
    console.log(obj.name);
    console.log(obj);
}

- -輸出 - -

1
"falana"
// [object Object] 
{
  "name": "falana",
  "count": 1
}

在這個例子中，我們可以在沒有任何ReferenceError情況下訪問另一個函數（在本例中為myFunction ）中的全局對象obj 。

我試圖創建一個單頁Twitter 克隆（僅限 DOM 操作）並不斷收到此錯誤。 Uncaught ReferenceError: Cannot access 'userData' before initialization at postMessage

---導致錯誤的Javascript---

window.onload = function() {
  console.clear();
}

const userData = {
  username: 'Chinmay Ghule',
  userhandle: generateUserhandle(this.username),
  userPostCount: 0
};

function generateUserhandle(userData) {
  const usernameArr = userData.username.split(" ");
  usernameArr.forEach(element => {
    element.toLowerCase();
  });

  return "@" + usernameArr.join("");
}

// posts message entered in #message-text to
// message-output-container.
function postMessage() {

  console.log(userData);

  // get message from #message-text.
  const message = document.getElementById('message-text').value;
  console.log(`message: ${message}`);

  // check for length.
  console.log(`message length: ${message.length}`);
  if (message.length === 0) {
    return;
  }

  // create new div.
  const card = document.createElement('div');
  const userInfo = document.createElement('div');
  const userMessage = document.createElement('div');

  const usernameSpan = document.createElement('span');
  const userhandleSpan = document.createElement('span');
  const beforeTimeDotSpan = document.createElement('span');
  const timeSpan = document.createElement('span');

  usernameSpan.classList.add('username');
  userhandleSpan.classList.add('userhandle');
  beforeTimeDotSpan.classList.add('before-time-dot');
  timeSpan.classList.add('time');

  userInfo.appendChild(usernameSpan);
  userInfo.appendChild(userhandleSpan);
  userInfo.appendChild(beforeTimeDotSpan);
  userInfo.appendChild(timeSpan);

  console.log(`userInfo : ${userInfo}`);

  userInfo.classList.add('user-info');
  userMessage.classList.add('output-message');

  card.appendChild(userInfo);
  card.appendChild(userMessage);

  console.log(`card : ${card}`);

  card.classList.add('output-message');

  userMessage.innerText = message;

  // check for number of posts.
  if (userData.userPostCount === 0) {
    let noMessageDiv = document.getElementById("no-message-display");

    noMessageDiv.remove();
  }

  // append new div.
  const messageOutputContainer = document.getElementById('message-output-container');

  messageOutputContainer.appendChild(card);

  // increment userPostCount.
  userData.userPostCount++;
}

為什么在這種情況下我會收到這個ReferenceError ，而在我們的第一個示例代碼中卻沒有？

Answer 1

你的代碼有很多問題......

這里最大的變化是完全擺脫了generateUserhandle並使用了getter 。

  get userhandle() {
    return this.username.toLowerCase()
  },

工作演示

 window.onload = function() { console.clear(); } const userData = { username: 'Chinmay Ghule', get userhandle() { return this.username.toLowerCase() }, userPostCount: 0 }; // posts message entered in #message-text to // message-output-container. function postMessage() { console.log(userData); // get message from #message-text. const message = document.getElementById('message-text').value; console.log(`message: ${message}`); // check for length. console.log(`message length: ${message.length}`); if (message.length === 0) { return; } // create new div. const card = document.createElement('div'); const userInfo = document.createElement('div'); const userMessage = document.createElement('div'); const usernameSpan = document.createElement('span'); const userhandleSpan = document.createElement('span'); const beforeTimeDotSpan = document.createElement('span'); const timeSpan = document.createElement('span'); usernameSpan.classList.add('username'); userhandleSpan.classList.add('userhandle'); beforeTimeDotSpan.classList.add('before-time-dot'); timeSpan.classList.add('time'); userInfo.appendChild(usernameSpan); userInfo.appendChild(userhandleSpan); userInfo.appendChild(beforeTimeDotSpan); userInfo.appendChild(timeSpan); console.log(`userInfo : ${userInfo}`); userInfo.classList.add('user-info'); userMessage.classList.add('output-message'); card.appendChild(userInfo); card.appendChild(userMessage); console.log(`card : ${card}`); card.classList.add('output-message'); userMessage.innerText = message; // check for number of posts. if (userData.userPostCount === 0) { let noMessageDiv = document.getElementById("no-message-display"); noMessageDiv.remove(); } // append new div. const messageOutputContainer = document.getElementById('message-output-container'); messageOutputContainer.appendChild(card); // increment userPostCount. userData.userPostCount++; }

 *, *::before, *::after { box-sizing: border-box; } body { margin: 0px; padding: 0px; font-family: Arial, Helvetica, sans-serif; } #container { /*background-color: lightskyblue;*/ margin: 2.5rem 25%; } #user-info { /*background-color: orange;*/ padding-bottom: 0.5rem; } .username { font-weight: bold; } .userhandle, .before-time-dot, .time { opacity: 0.75; } #message-text { width: 100%; margin-bottom: 0.5rem; font-size: 18px; padding: 0.5rem; resize: none; border-radius: 0.5rem; outline: 1px solid lightgray; } #message-button { float: right; padding: 0.375rem 1.5rem; font-weight: bold; font-size: 18px; border-radius: 1rem; } #message-button:hover { background-color: lightgray; } #message-input-container { background-color: lightskyblue; padding: 1rem; border-radius: 0.5rem; } #message-input-container::after { content: ""; clear: both; display: table; } #message-output-container { /*background-color: lightskyblue;*/ margin-top: 30px; border-top: 3px solid black; } #no-message-display { text-align: center; padding: 0.5rem; } .output-message { padding: 0.5rem; font-size: 18px; border-bottom: 1px solid lightgray; }

 <div id="container"> <div id="message-input-container"> <textarea id="message-text" rows="5" placeholder="Type your message here..." contenteditable="" value=""></textarea> <input id="message-button" type="button" value="Post" onclick="postMessage();" /> </div> <div id="message-output-container"> <div id="no-message-display"> <span>No messages yet!</span> </div> </div> </div>