JavaScript has no character type. Strings are primitive types, so they are immutable. They are zero-indexed, can be accessed using subscript notation 'foo'[0] and have a length property provided by the String wrapper object. An empty string has a length of 0. Concatenation is performed by +. String literals can be continued across a line break in source code using \.

JavaScript strings are UTF-16 encoded, and each code unit is represented by a 16-bit pattern. For this reason, the length property of a string returns its length in code units, not characters; if your language uses two-unit characters, you’re not going to get an accurate character length:

> "🔥".length
2

The same applies to string indexing, using either charAt() or subscript notation:

> "🔥💡".charAt(0)
"�"
> "🔥💡"[0]
"�"

The same applies to string-manipulation methods.

for .. of, a relatively recent feature, will operate on full characters, not code units:

> for (const char of "🔥💡") console.log(char)
🔥
💡

Similarly, while charCodeAt() will return a code unit, not a full character code, you can get the latter with codePointAt(), introduced after problems with UTF-16 had been absorbed.

String object methods include Java-style charAt(); indexOf() and lastIndexOf(); substring() and slice(); split(); case conversion methods; and search(), match(), and replace(). Because strings are immutable, mutator methods return a new string.

Sources

Flanagan, David. Javascript: The Definitive Guide. 6th ed, O’Reilly, 2011.

Haverbeke, Marijn. Eloquent Javascript: A Modern Introduction to Programming. http://eloquentjavascript.net.