当前位置：文江博客知识库 en-US 文档 Learn JavaScript Howto

Solve common problems in your JavaScript code - Learn web development 编辑

The following links point to solutions to common problems you may encounter when writing JavaScript.

Common beginner's mistakes

Correct spelling and casing

If your code doesn't work and/or the browser complains that something is undefined, check that you've spelt all your variable names, function names, etc. correctly.

Some common built-in browser functions that cause problems are:

Correct	Wrong
`getElementsByTagName()`	`getElementbyTagName()`
`getElementsByName()`	`getElementByName()`
`getElementsByClassName()`	`getElementByClassName()`
`getElementById()`	`getElementsById()`

Semi-colon position

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;'`

Functions

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 with the following statement:

myFunction();

Function scope

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 from the function.

Running code after a return statement

Remember also that when you return from a function, the JavaScript interpreter exits the function — no code 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".

Object notation versus normal assignment

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

const myNumber = 0;

With Objects, however, you need to take care to use the correct syntax. The object must be surrounded by curly braces, member names must be separated from their values using colons, and members must be separated by commas. For example:

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

Basic definitions

Basic use cases

General

Variables

Math

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).

Strings

Arrays

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 do you add and remove array items?
How do you split a string into array items, or join array items into a string?

Debugging JavaScript

For more information on JavaScript debugging, see Handling common JavaScript problems. Also, see Other common errors for a description of common errors.