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
- How do you declare a variable?
- How do you initialize a variable with a value?
- How do you update a variable's value? (also see Assignment operators)
- What data types can values have in JavaScript?
- What does 'loosely typed' mean?
數學運算
- What types of number do you have to deal with in web development?
- How do you do basic math in JavaScript?
- What is operator precedence, and how is it handled in JavaScript?
- How do you increment and decrement values in JavaScript?
- How do you compare values in JavaScript? (e.g. to see which one is bigger, or to see if one value is equal to another).
字串
- How do you create a string in JavaScript?
- Do you have to use single quotes or double quotes?
- How do you escape characters in strings?
- How do you join strings together?
- Can you join strings and numbers together?
- How do you find the length of a string?
- How you find what character is at a certain position in a string?
- How do you find and extract a specific substring from a string?
- How do you change the case of a string?
- How do you replace one specific substring with another?
序列
- How do you create an array?
- How do you access and modify the items in an array? (this includes multidimensional arrays)
- How do you find the length of an array?
- How you add and remove array items?
- How do you split a string into array items, or join array items into a string?
Debugging JavaScript
- What are the basic types of error?
- What are browser developer tools, and how do you access them?
- How do you log a value to the JavaScript console?
- How do you use breakpoints, and other JavaScript debugging features?
For more information on JavaScript debugging, see Handling common JavaScript problems; also see Other common errors for a description of common errors.
Making decisions in code
- How do you execute different blocks of code, depending on a variable's value or other condition?
- How do you use if ...else statements?
- How do nest one decision block inside another?
- How do you use AND, OR, and NOT operators in JavaScript?
- How do you conveniently handle a large number of choices for one condition?
- How do you use a ternary operator to make a quick choice between two options based on a true or false test?
Looping/iteration
- How do you run the same bit of code over and over again?
- How do you exit a loop before the end if a certain condition is met?
- How do you skip to the next iteration of a loop if a certain condition is met?
- How do you use while and do ... while loops?
- How to iterate over the elements in an array
- How to iterate over the elements in a multidimensional array
- How to iterate over the members in an object
- How to iterate over the members of an object nested inside an array
進階使用例子
Functions
- How do you find functions in the browser?
- What is the difference between a function and a method?
- How do you create your own functions?
- How do you run (call, or invoke) a function?
- What is an anonymous function?
- How do you specify parameters (or arguments) when invoking a function?
- What is function scope?
- What are return values, and how do you use them?
對向
- How do you create an object?
- What is dot notation?
- What is bracket notation?
- How do you get and set the methods and properties of an object?
- What is
this
, in the context of an object? - What is object-oriented programming?
- What are constructors and instances, and how do you create them?
- What different ways are there to create objects in JavaScript?
JSON
任務
- What are event handlers and how do you use them?
- What are inline event handlers?
- What does the
addEventListener()
function do, and how do you use it? - Which mechanism should I use to add event code to my web pages?
- What are event objects, and how do you use them?
- How do you prevent default event behaviour?
- How do events fire on nested elements? (event propagation, also related — event bubbling and capturing)
- What is event delegation, and how does it work?
Object-oriented JavaScript
- What are object prototypes?
- What is the constructor property, and how can you use it?
- How do you add methods to the constructor?
- How do you create a new constructor that inherits its members from a parent constructor?
- When should you use inheritance in JavaScript?
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