You Don't Know JS Yet: Types & Grammar - 2nd Edition

Chapter 1: Types

Work in progress








Everything below here is previous text from 1st edition, and is only here for reference while 2nd edition work is underway. Please ignore this stuff.

Most developers would say that a dynamic language (like JS) does not have types. Let's see what the ES5.1 specification ( has 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 sub classified 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 may 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 claim to have "types," and they should instead be called "tags" or perhaps "subtypes".

Bah! We're going to use this rough 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 the behavior of 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 -- 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 this discussion. And it's consistent with how JS describes itself.

A Type By Any Other Name...

Beyond academic definition disagreements, why does it matter if JavaScript has types or not?

Having a proper understanding of each type and its intrinsic behavior is absolutely essential to understanding how to properly and accurately convert values to different types (see Coercion, Chapter 4). Nearly every JS program ever written will need to handle value coercion in some shape or form, so it's important you do so responsibly and with confidence.

If you have the number value 42, but you want to treat it like a string, such as pulling out the "2" as a character in position 1, you obviously must first convert (coerce) the value from number to string.

That seems simple enough.

But there are many different ways that such coercion can happen. Some of these ways are explicit, easy to reason about, and reliable. But if you're not careful, coercion can happen in very strange and surprising ways.

Coercion confusion is perhaps one of the most profound frustrations for JavaScript developers. It has often been criticized as being so dangerous as to be considered a flaw in the design of the language, to be shunned and avoided.

Armed with a full understanding of JavaScript types, we're aiming to illustrate why coercion's bad reputation is largely overhyped and somewhat undeserved -- to flip your perspective, to seeing coercion's power and usefulness. But first, we have to get a much better grip on values and types.

Built-in Types

JavaScript defines seven built-in types:

  • null
  • undefined
  • boolean
  • number
  • string
  • object
  • symbol -- added in ES6!

Note: All of these types except object are called "primitives".

The typeof operator inspects the type of the given value, and always returns one of seven string values -- surprisingly, there's not an exact 1-to-1 match with the seven built-in types we just listed.

typeof undefined     === "undefined"; // true
typeof true          === "boolean";   // true
typeof 42            === "number";    // true
typeof "42"          === "string";    // true
typeof { life: 42 }  === "object";    // true

// added in ES6!
typeof Symbol()      === "symbol";    // true

These six listed types have values of the corresponding type and return a string value of the same name, as shown. Symbol is a new data type as of ES6, and will be covered in Chapter 3.

As you may have noticed, I excluded null from the above listing. It's special -- special in the sense that it's buggy when combined with the typeof operator:

typeof null === "object"; // true

It would have been nice (and correct!) if it returned "null", but this original bug in JS has persisted for nearly two decades, and will likely never be fixed because there's too much existing web content that relies on its buggy behavior that "fixing" the bug would create more "bugs" and break a lot of web software.

If you want to test for a null value using its type, you need a compound condition:

var a = null;

(!a && typeof a === "object"); // true

null is the only primitive value that is "falsy" (aka false-like; see Chapter 4) but that also returns "object" from the typeof check.

So what's the seventh string value that typeof can return?

typeof function a(){ /* .. */ } === "function"; // true

It's easy to think that function would be a top-level built-in type in JS, especially given this behavior of the typeof operator. However, if you read the spec, you'll see it's actually a "subtype" of object. Specifically, a function is referred to as a "callable object" -- an object that has an internal [[Call]] property that allows it to be invoked.

The fact that functions are actually objects is quite useful. Most importantly, they can have properties. For example:

function a(b,c) {
    /* .. */

The function object has a length property set to the number of formal parameters it is declared with.

a.length; // 2

Since you declared the function with two formal named parameters (b and c), the "length of the function" is 2.

What about arrays? They're native to JS, so are they a special type?

typeof [1,2,3] === "object"; // true

Nope, just objects. It's most appropriate to think of them also as a "subtype" of object (see Chapter 3), in this case with the additional characteristics of being numerically indexed (as opposed to just being string-keyed like plain objects) and maintaining an automatically updated .length property.

Values as Types

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 doesn't have "type enforcement," in that the engine doesn't insist that a variable always holds values of the same initial type that it starts out with. A variable can, in one assignment statement, hold a string, and in the next hold a number, and so on.

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 (see Chapter 4).

If you use typeof against a variable, it's not asking "what's the type of the variable?" as it may seem, since JS variables have no types. Instead, it's asking "what's the type of the value in the variable?"

var a = 42;
typeof a; // "number"

a = true;
typeof a; // "boolean"

The typeof operator always returns a string. So:

typeof typeof 42; // "string"

The first typeof 42 returns "number", and typeof "number" is "string".

undefined vs "undeclared"

Variables that have no value currently, actually have the undefined value. Calling typeof against such variables will return "undefined":

var a;

typeof a; // "undefined"

var b = 42;
var c;

// later
b = c;

typeof b; // "undefined"
typeof c; // "undefined"

It's tempting for most developers to think of the word "undefined" and think of it as a synonym for "undeclared." However, in JS, these two concepts are quite different.

An "undefined" variable is one that has been declared in the accessible scope, but at the moment has no other value in it. By contrast, an "undeclared" variable is one that has not been formally declared in the accessible scope.


var a;

a; // undefined
b; // ReferenceError: b is not defined

An annoying confusion is the error message that browsers assign to this condition. As you can see, the message is "b is not defined," which is of course very easy and reasonable to confuse with "b is undefined." Yet again, "undefined" and "is not defined" are very different things. It'd be nice if the browsers said something like "b is not found" or "b is not declared," to reduce the confusion!

There's also a special behavior associated with typeof as it relates to undeclared variables that even further reinforces the confusion. Consider:

var a;

typeof a; // "undefined"

typeof b; // "undefined"

The typeof operator returns "undefined" even for "undeclared" (or "not defined") variables. Notice that there was no error thrown when we executed typeof b, even though b is an undeclared variable. This is a special safety guard in the behavior of typeof.

Similar to above, it would have been nice if typeof used with an undeclared variable returned "undeclared" instead of conflating the result value with the different "undefined" case.

typeof Undeclared

Nevertheless, this safety guard is a useful feature when dealing with JavaScript in the browser, where multiple script files can load variables into the shared global namespace.

Note: Many developers believe there should never be any variables in the global namespace, and that everything should be contained in modules and private/separate namespaces. This is great in theory but nearly impossible in practicality; still it's a good goal to strive toward! Fortunately, ES6 added first-class support for modules, which will eventually make that much more practical.

As a simple example, imagine having a "debug mode" in your program that is controlled by a global variable (flag) called DEBUG. You'd want to check if that variable was declared before performing a debug task like logging a message to the console. A top-level global var DEBUG = true declaration would only be included in a "debug.js" file, which you only load into the browser when you're in development/testing, but not in production.

However, you have to take care in how you check for the global DEBUG variable in the rest of your application code, so that you don't throw a ReferenceError. The safety guard on typeof is our friend in this case.

// oops, this would throw an error!
if (DEBUG) {
    console.log( "Debugging is starting" );

// this is a safe existence check
if (typeof DEBUG !== "undefined") {
    console.log( "Debugging is starting" );

This sort of check is useful even if you're not dealing with user-defined variables (like DEBUG). If you are doing a feature check for a built-in API, you may also find it helpful to check without throwing an error:

if (typeof atob === "undefined") {
    atob = function() { /*..*/ };

Note: If you're defining a "polyfill" for a feature if it doesn't already exist, you probably want to avoid using var to make the atob declaration. If you declare var atob inside the if statement, this declaration is hoisted (see the Scope & Closures title of this series) to the top of the scope, even if the if condition doesn't pass (because the global atob already exists!). In some browsers and for some special types of global built-in variables (often called "host objects"), this duplicate declaration may throw an error. Omitting the var prevents this hoisted declaration.

Another way of doing these checks against global variables but without the safety guard feature of typeof is to observe that all global variables are also properties of the global object, which in the browser is basically the window object. So, the above checks could have been done (quite safely) as:

if (window.DEBUG) {
    // ..

if (!window.atob) {
    // ..

Unlike referencing undeclared variables, there is no ReferenceError thrown if you try to access an object property (even on the global window object) that doesn't exist.

On the other hand, manually referencing the global variable with a window reference is something some developers prefer to avoid, especially if your code needs to run in multiple JS environments (not just browsers, but server-side node.js, for instance), where the global object may not always be called window.

Technically, this safety guard on typeof is useful even if you're not using global variables, though these circumstances are less common, and some developers may find this design approach less desirable. Imagine a utility function that you want others to copy-and-paste into their programs or modules, in which you want to check to see if the including program has defined a certain variable (so that you can use it) or not:

function doSomethingCool() {
    var helper =
        (typeof FeatureXYZ !== "undefined") ?
        FeatureXYZ :
        function() { /*.. default feature ..*/ };

    var val = helper();
    // ..

doSomethingCool() tests for a variable called FeatureXYZ, and if found, uses it, but if not, uses its own. Now, if someone includes this utility in their module/program, it safely checks if they've defined FeatureXYZ or not:

// an IIFE (see "Immediately Invoked Function Expressions"
// discussion in the *Scope & Closures* title of this series)
    function FeatureXYZ() { /*.. my XYZ feature ..*/ }

    // include `doSomethingCool(..)`
    function doSomethingCool() {
        var helper =
            (typeof FeatureXYZ !== "undefined") ?
            FeatureXYZ :
            function() { /*.. default feature ..*/ };

        var val = helper();
        // ..


Here, FeatureXYZ is not at all a global variable, but we're still using the safety guard of typeof to make it safe to check for. And importantly, here there is no object we can use (like we did for global variables with window.___) to make the check, so typeof is quite helpful.

Other developers would prefer a design pattern called "dependency injection," where instead of doSomethingCool() inspecting implicitly for FeatureXYZ to be defined outside/around it, it would need to have the dependency explicitly passed in, like:

function doSomethingCool(FeatureXYZ) {
    var helper = FeatureXYZ ||
        function() { /*.. default feature ..*/ };

    var val = helper();
    // ..

There are lots of options when designing such functionality. No one pattern here is "correct" or "wrong" -- there are various tradeoffs to each approach. But overall, it's nice that the typeof undeclared safety guard gives us more options.


JavaScript has seven built-in types: null, undefined, boolean, number, string, object, symbol. They can be identified by the typeof operator.

Variables don't have types, but the values in them do. These types define intrinsic behavior of the values.

Many developers will assume "undefined" and "undeclared" are roughly the same thing, but in JavaScript, they're quite different. undefined is a value that a declared variable can hold. "Undeclared" means a variable has never been declared.

JavaScript unfortunately kind of conflates these two terms, not only in its error messages ("ReferenceError: a is not defined") but also in the return values of typeof, which is "undefined" for both cases.

However, the safety guard (preventing an error) on typeof when used against an undeclared variable can be helpful in certain cases.

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