JavaScript 解決常見的問題

以下鏈接針對您需要修復的常見問題的解決方案，以便讓您的 JavaScript 語法正確執行。

初學者常見的錯誤

糾正語法和代碼

如果您的代碼毫無反映或瀏覽器反饋某些內容「未定義」，請檢查您是否「正確輸入」所有變量名稱，函數名稱等。

以下的常見造成問題的內置瀏覽器功能比對：

正確 錯誤
getElementsByTagName() getElementbyTagName()
getElementsByName() getElementByName()
getElementsByClassName() getElementByClassName()
getElementById() getElementsById()

分號位置

You need to make sure you don't place any semi-colons incorrectly. For example:

Correct Wrong
elem.style.color = 'red'; elem.style.color = 'red;'

功能內容

There are a number of things that can go wrong with functions.

One of the most common errors is to declare the function, but not call it anywhere. For example:

function myFunction() {
  alert('This is my function.');
};

This code won't do anything unless you call it, for example with

myFunction();

功能範圍

Remember that functions have their own scope — you can't access a variable value set inside a function from outside the function, unless you declared the variable globally (i.e. not inside any functions), or return the value out of the function.

在 return 語句後執行語法

Remember also that when you return a value out of a function, the JavaScript interpreter exits the function — no code declared after the return statement will run.

In fact, some browsers (like Firefox) will give you an error message in the developer console if you have code after a return statement. Firefox gives you "unreachable code after return statement".

對象表示法與正確的指派

When you assign something normally in JavaScript, you use a single equals sign, e.g.:

const myNumber = 0;

This doesn't work in Objects, however — with objects you need to separate member names from their values using colons, and separate each member with a comma, for example:

const myObject = {
  name: 'Chris',
  age: 38
}

基本定義

基本使用例子

概括

變量

數學運算

字串

序列

Debugging JavaScript

For more information on JavaScript debugging, see Handling common JavaScript problems (en-US); also see Other common errors for a description of common errors.

Making decisions in code

Looping/iteration

進階使用例子

Functions

對向

JSON

任務

Object-oriented JavaScript

網頁 Web APIs