Update 2016 - five years on there are now new methods in the specs (see support below) to convert between strings and typed arrays using proper encoding.

TextEncoder

The TextEncoder represents:

The TextEncoder interface represents an encoder for a specific method, that is a specific character encoding, like utf-8, iso-8859-2, koi8, cp1261, gbk, ... An encoder takes a stream of code points as input and emits a stream of bytes.

Change note since the above was written: (ibid.)

Note: Firefox, Chrome and Opera used to have support for encoding types other than utf-8 (such as utf-16, iso-8859-2, koi8, cp1261, and gbk). As of Firefox 48 [...], Chrome 54 [...] and Opera 41, no other encoding types are available other than utf-8, in order to match the spec.*

*) Updated specs (W3) and here (whatwg).

After creating an instance of the TextEncoder it will take a string and encode it using a given encoding parameter:

if (!("TextEncoder" in window)) 
  alert("Sorry, this browser does not support TextEncoder...");

var enc = new TextEncoder(); // always utf-8
console.log(enc.encode("This is a string converted to a Uint8Array"));

You then of course use the .buffer parameter on the resulting Uint8Array to convert the underlaying ArrayBuffer to a different view if needed.

Just make sure that the characters in the string adhere to the encoding schema, for example, if you use characters outside the UTF-8 range in the example they will be encoded to two bytes instead of one.

For general use you would use UTF-16 encoding for things like localStorage.