Accessibility is a key principle within usability that states the importance of catering to all users, regardless of their abilities and circumstances. Usability tends to concern itself with the user as if they are a single entity. We usually make specific assumptions about the user, bestowing them with a set of characteristics and capabilities that may not be reflected in reality. Accessibility, however, is about the real users who will end up having to use whatever you've created. These real users are a diverse set of individuals and may have all manner of differences. When we talk about accessibility in software, we are usually concerned with types of differences that directly affect a person's ability to make use of that software. These may include the follows:

• Learning disorders or differences such as dyslexia.
• Physical disabilities. For example, limited mobility of the hands or blindness.
• Developmental disorders such as Autism and ADHD.
• Less access to technology due to mobility, economy, or infrastructure.

In addition to these, there are many other differences that span the gamut of human existence, so we should always be ready to learn and adapt according to new needs and differences that we encounter among our users.

We are engaged in the creation of web applications, both on the server and in the browser. As JavaScript programmers, we sit very close to the interfaces that are served to end users. Therefore, it's vital that we have an excellent grasp of accessibility on the web. This includes an awareness of the Web Content Accessibility Guidelines (WCAG 2.0) published by the W3C, which include the following provisions:

• Provide text alternatives for any non-text content (guideline 1.1)
• Make all functionality available from a keyboard (guideline 2.1)
• Make text content readable and understandable (guideline 3.1)

Accessibility is not only about the non-programmer end user. As mentioned, we should consider other programmers to also be our users in the same sense as the end users of GUIs or other web interfaces. It is vital that we cater to other programmers. Some programmers are blind or partially sighted. Some programmers have learning or cognitive difficulties. Not all programmers work on the latest and fastest hardware. Neither do all programmers understand all the things you may take for granted. It's important to take all of these things into consideration in the code we write. 

Having now finished this chapter, you may have a sense of feeling overwhelmed by the number of tenets, principles, and guidelines. Things may seem complex, but they aren't if we follow one simple rule—always focus on the user. Also, remember that other programmers who may work on your code are your users as well.

As programmers, we sit in a position where we wield unprecedented power in helping to define the behaviors that people conduct in the execution of all manner of tasks. The original programmers who worked at Twitter, Google, or Microsoft likely did not foresee the number of times their code would run. They probably couldn't have originally imagined how many humans their code would end up affecting. We should always remain humble to this power and try in earnest to be accountable to all of the users we serve, and all the myriad tasks they seek to carry out. If you were to come away with one thing from this chapter, I hope it would be simply this: a humble and continued consideration of the user, in every line of code you write.