Most developers would tell you that a dynamic language (like JS) does not have types. Let's see what the authors of the ES5.1 specification have to say on the topic:
Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification types.
An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Number, and Object.
Now, if you're a fan of strongly-typed (statically-typed) languages, you probably object to this usage of the word "type". In those languages, "type" means a whole lot more than it does here in JS.
Some people say JS shouldn't be said to have "types", but they should instead be called "tags" or perhaps "sub types".
Bah. We're going to use this definition (the same one that seems to drive the wording of the spec!): a type is an intrinsic, built-in set of characteristics that uniquely identifies a particular value and distinguishes it from other values, both to the engine and to the developer.
In other words, if both the engine and the developer treat value 42 (the number) differently than they treat value "42" (the string), then those two values have different types, namely number and string, respectively. When you use 42, you are intending to do something numeric, like math. But when you use "42", you are intending to do something string'ish, like outputting to the page, etc. These two values have different types.
That's by no means a perfect definition. But it's good enough for now. And it's consistent with how JS treats itself.
In JavaScript, variables don't have types -- values have types. Variables can hold any value, at any time.
Another way to think about JS types is that JS has types but it doesn't have "type enforcement", in that the engine doesn't insist that variable maintains values that are all of the same initial type. A variable can, in one statement, hold a string, and in the next hold a number, and so on.
Also, the value 42 has an intrinsic type of number, and its type cannot be changed. Another value, like "42", with the string type, can be created from the number value 42, through a process called coercion, which we will cover later.