Casting to string in JavaScript

They do behave differently when the value is null.

• null.toString() throws an error - Cannot call method 'toString' of null
• String(null) returns - "null"
• null + "" also returns - "null"

Very similar behaviour happens if value is undefined (see jbabey's answer).

Other than that, there is a negligible performance difference, which, unless you're using them in huge loops, isn't worth worrying about.

There are differences, but they are probably not relevant to your question. For example, the toString prototype does not exist on undefined variables, but you can cast undefined to a string using the other two methods:

​var foo;

​var myString1 = String(foo); // "undefined" as a string

var myString2 = foo + ''; // "undefined" as a string

var myString3 = foo.toString(); // throws an exception

http://jsfiddle.net/f8YwA/