JS101: A Primer on Strings and String Encodings

2012-12-31 00:00:00 +0000 by Alex R. Young

What is a JavaScript string? It depends on the context. For instance, a string is a primitive value -- a value represented at the "lowest level" of the language's implementation.

Strings are also members of the type String. Strings can be created with the String constructor. Running new String('hello') creates an instance of String.

Finally, String literals are found in the program's source: var name = 'alex'.

Given that there are many ways to represent strings, what is the underlying encoding in JavaScript? Both the third and fifth editions of ECMAScript state that strings are represented as 16-bit unsigned integers:

Each integer value in the sequence usually represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the values except that they must be 16-bit unsigned integers.

String Encoding

Ultimately a string is just a sequence of characters. In other words, an array of units of information that correspond to digits, letters, and so on. Characters are represented as byte sequences.

When working on client-side JavaScript and HTML, we're used to seeing charset=UTF-8. UTF-8 is a system for encoding characters, and is actually "variable width", which means the bytes used to represent an individual character can vary in length.

I said earlier that JavaScript strings are 16-bit, so how does this relate to UTF-8? In extremely simplified terms for the purposes of a beginner's article, you can think about it like this: JavaScript engines use a fixed 16-bit representation of characters that makes it easier to manage strings internally.

So, even though a browser's JavaScript engine internally represents characters as 16-bit numbers, we don't usually need to know about this. Writing the strings to form controls with the DOM or using XMLHTTPRequest should convert the string to the right encoding. Ideally the server should have sent the Content-Type header set to UTF-8, so the browser will know what to do.

More About Encodings

Even if you're a client-side developer that doesn't care about string encodings, Joel Spolsky's The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets is worth reading because it explains the history behind string encodings. Understanding the history behind what can be a frustrating topic makes it easier to understand.

If you need to work on string encodings in JavaScript, Johan Sundström's post Encoding / decoding UTF8 in javascript from back in 2006 explains how to encode and decode UTF-8.

Monsur Hossain went on to write UTF-8 in JavaScript which goes into unescape and encodeURIComponent in more detail.