Javascript is a loosely typed language. That means a variable x may now hold a number, two lines down a string and another ten lines down an HTML element. What might be confusing is that Javascript often converts types automatically. The rules behind automatic type convertion sometimes are – well, surprisingly abnormal. Did you know that

"0" == false

evaluates to true and why?

Programmers from other languages such as Java often find dynamic typing challenging, and therefore try to twist Javascript to help avoid typing problems like that:

function isString(obj) { return typeof(obj) == 'string'; } function isNumber(obj) { return typeof(obj) == 'number'; } function isBoolean(obj) { return typeof(obj) == 'boolean'; } function isFunction(obj) { return typeof(obj) == 'function'; } function isObject(obj) { return typeof(obj) == 'object' || isFunction(obj); } function isArray(obj) { return isObject(obj) && obj instanceof Array; } function isDate(obj) { return isObject(obj) && obj instanceof Date; } function isError(obj) { return isObject(obj) && obj instanceof Error; } function isUndefined(obj) { return typeof(obj) == 'undefined'; } function isNull(obj) { return obj === null; } function isNone(obj) { return isUndefined(obj) || isNull(obj); } function isSet(obj) { return !isNone(obj); } function isTrue(obj) { return isSet(obj) && !!obj; } function isFalse(obj) { return isSet(obj) && !obj; }

That’s a real life example. I think it’s a pity. Though it might help in the first hand, it hinders the developer to truely master the language and the benefits that come with dynamic typing.

There are four important aspects involved with dynamic typing, which I will cover in a series of articles.

The first (the one you are reading) includes the basics: What types does Javascript have and what are the general rules of type convertion.

The second is about automatic type convertion. When and how does automatic type convertion occur?

Sometimes you need to explicitly convert a type to another. The third part will cover the most suitable ways to do so.

Finally we consider ways of type detection. For some types that’s easy, for others it spells disaster.

Types in Javascript – The basics

There are five primitive types in Javascript and there are objects. The primitive types are undefined, null, boolean, number, and string. Objects include (but are not limited to) Array, Function, and Date.

The type undefined has exactly one value: undefined. It is used when a variable has not been assigned a value.

var a; // typeof a is "undefined"

The type null has exactly one value as well: null. It is mostly used to represent a non-existent reference.

var b = document.getElementById("thisIdIsNotWithinTheDocument"); // b is null

We slowly increase the number of values. The type boolean has to values: true and false.

var c = true; var d = !c; // false

Now there is a huge step in values. There are 18437736874454810627 possible values for the number type. Numbers in Javascript are represented by double-precision 64-bits according to IEEE 754.

Take a moment and think about it. You know what I mean? There are no integers in javascript! Though integers between -231 and 231 – 1 are represented exactly in Javascript they still are floating point numbers, not integers.

In addition, there are three special values: NaN, which stands for “not a number” (though still it is a number), positive Infinity and negative Infinity.

var e = 42; var f = e / 0; // f now is Infinity

As a footnote I want to add that there are two zeros in Javascript. You rarely will encounter a situation where that matters. I have yet to make that experience.

A string in Javascript is a primitive in contrast to e.g. Java. Thus you compare two strings by the equality operator ===, you do not need an equals function. Every character is represented by a 16-bit unsigned integer value, therefore unicode characters with codepoint 65536 and above cannot be used in Javascript.

var g = "hello world";

Everything else is an object. For type convertion it does not matter which kind of an object you have. Mostly. I will cover the details in the coming sections.

Conversion

Some statements in javascript and many operators expect values of a specific type. The if statement for example expects a boolean and if you call it with any other type it has to be converted.

Conversion to boolean

Convertion to boolean is quite easy. Null and undefined are converted to false, any object is converted to true. Numbers are converted to true except for 0 and NaN which are converted to false. The empty string is converted to false, any other to true.