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Use JavaScript to solve common problems

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The following links point to solutions to common everyday problems you'll need to solve with 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.   

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 semi-colon 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.');
};

Won't do anything unless you call it, for example with

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 out of the function.

Running code after a return statement

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".

Object notation versus normal assignment

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

var 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:

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

Basic definitions

Basic use cases

Arrays

Debugging JavaScript

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

Looping/iteration

Intermediate use cases

Document Tags and Contributors

 Contributors to this page: adekoyejo, chrisdavidmills
 Last updated by: adekoyejo,