JavaScript has only six types, according to Chapter 8 of the ECMAScript language specification:

Therefore, constructors technically don’t introduce new types, even though they are said to have instances.

In the context of language semantics and type systems, static usually means “at compile time” or “without running a program,” while dynamic means “at runtime.”

In a statically typed language, variables, parameters, and members of objects (JavaScript calls them properties) have types that the compiler knows at compile time. The compiler can use that information to perform type checks and to optimize the compiled code.

Even in statically typed languages, a variable also has a dynamic type, the type of the variable’s value at a given point at runtime. The dynamic type can differ from the static type. For example (Java):

Object foo = "abc";

The static type of foo is Object; its dynamic type is String.

JavaScript is dynamically typed; types of variables are generally not known at compile time.

If you have type information, you can check whether a value used in an operation (calling a function, applying an operator, etc.) has the correct type. Statically type-checked languages perform this kind of check at compile time, while dynamically type-checked languages do so at runtime. A language can be both statically type-checked and dynamically type-checked. If a check fails, you usually get some kind of error or exception.

JavaScript performs a very limited kind of dynamic type checking:

> var foo = null;
> foo.prop
TypeError: Cannot read property 'prop' of null

Mostly, however, things silently fail or work. For example, if you access a property that does not exist, you get the value undefined:

> var bar = {};
> bar.prop
undefined

In JavaScript, the main way of dealing with a value whose type doesn’t fit is to coerce it to the correct type. Coercion means implicit type conversion. Most operands coerce:

> '3' * '4'
12

JavaScript’s built-in conversion mechanisms support only the types Boolean, Number, String, and Object. There is no standard way to convert an instance of one constructor to an instance of another constructor.

Here we review the various scenarios where undefined and null occur.

> var foo;
> foo
undefined

> function f(x) { return x }
> f()
undefined

> var obj = {}; // empty object
> obj.foo
undefined

> function f() {}
> f()
undefined

> function g() { return; }
> g()
undefined

In the following sections we review how to check for undefined and null individually, or to check if either exists.

if (x === null) ...

if (x === undefined) ...

// Does x have a value?
if (x !== undefined && x !== null) {
    ...
}
// Is x a non-value?
if (x === undefined || x === null) {
    ...
}

// Does x have a value (is it truthy)?
if (x) {
    ...
}
// Is x falsy?
if (!x) {
    ...
}

A single nonvalue could play the roles of both undefined and null. Why does JavaScript have two such values? The reason is historical.

JavaScript adopted Java’s approach of partitioning values into primitives and objects. It also used Java’s value for “not an object,” null. Following the precedent set by C (but not Java), null becomes 0 if coerced to a number:

> Number(null)
0
> 5 + null
5

Remember that the first version of JavaScript did not have exception handling. Therefore, exceptional cases such as uninitialized variables and missing properties had to be indicated via a value. null would have been a good choice, but Brendan Eich wanted to avoid two things at the time:

As a result, Eich added undefined as an additional nonvalue to the language. It coerces to NaN:

> Number(undefined)
NaN
> 5 + undefined
NaN

undefined is a property of the global object (and thus a global variable; see The Global Object). Under ECMAScript 3, you had to take precautions when reading undefined, because it was easy to accidentally change its value. Under ECMAScript 5, that is not necessary, because undefined is read-only.

To protect against a changed undefined, two techniques were popular (they are still relevant for older JavaScript engines):

Primitive values such as 'abc' are fundamentally different from wrapper instances such as new String('abc'):

> typeof 'abc'  // a primitive value
'string'
> typeof new String('abc')  // an object
'object'
> 'abc' instanceof String  // never true for primitives
false
> 'abc' === new String('abc')
false

Wrapper instances are objects, and there is no way of comparing objects in JavaScript, not even via lenient equals == (see Equality Operators: === Versus ==):

> var a = new String('abc');
> var b = new String('abc');
> a == b
false

There is one use case for wrapper objects: you want to add properties to a primitive value. Then you wrap the primitive and add properties to the wrapper object. You need to unwrap the value before you can work with it.

Wrap a primitive by invoking a wrapper constructor:

new Boolean(true)
new Number(123)
new String('abc')

Unwrap a primitive by invoking the method valueOf(). All objects have this method (as discussed in Conversion to Primitive):

> new Boolean(true).valueOf()
true
> new Number(123).valueOf()
123
> new String('abc').valueOf()
'abc'

Converting wrapper objects to primitives properly extracts numbers and strings, but not booleans:

> Boolean(new Boolean(false))  // does not unwrap
true
> Number(new Number(123))  // unwraps
123
> String(new String('abc'))  // unwraps
'abc'

The reason for this is explained in Converting to Boolean.

Primitives don’t have their own methods and borrow them from wrappers:

> 'abc'.charAt === String.prototype.charAt
true

Sloppy mode and strict mode handle this borrowing differently. In sloppy mode, primitives are converted to wrappers on the fly:

String.prototype.sloppyMethod = function () {
    console.log(typeof this); // object
    console.log(this instanceof String); // true
};
''.sloppyMethod(); // call the above method

In strict mode, methods from the wrapper prototype are used transparently:

String.prototype.strictMethod = function () {
    'use strict';
    console.log(typeof this); // string
    console.log(this instanceof String); // false
};
''.strictMethod(); // call the above method

Therefore, JavaScript rarely complains about a value having the wrong type. For example, programs normally receive user input (from online forms or GUI widgets) as strings, even if the user has entered a number. If you treat a number-as-string like a number, you will not get a warning, just unexpected results. For example:

var formData = { width: '100' };

// You think formData.width is a number
// and get unexpected results
var w = formData.width;
var outer = w + 20;

// You expect outer to be 120, but it’s not
console.log(outer === 120);  // false
console.log(outer === '10020');  // true

In cases such as the preceding one, you should convert to the appropriate type early on:

var w = Number(formData.width);

The following functions are the preferred way of converting a value to a boolean, number, string, or object:

Note that Boolean(), Number(), String(), and Object() are called as functions. You normally don’t use them as constructors. Then they create instances of themselves (see Wrapper Objects for Primitives).

To convert a value to either a number or a string, it is first converted to an arbitrary primitive value, which is then converted to the final type (as discussed in Functions for Converting to Boolean, Number, String, and Object).

The ECMAScript specification has an internal function, ToPrimitive() (which is not accessible from JavaScript), that performs this conversion. Understanding ToPrimitive() enables you to configure how objects are converted to numbers and strings. It has the following signature:

ToPrimitive(input, PreferredType?)

The optional parameter PreferredType indicates the final type of the conversion: it is either Number or String, depending on whether the result of ToPrimitive() will be converted to a number or a string.

If PreferredType is Number, then you perform the following steps:

If PreferredType is String, steps 2 and 3 are swapped. The PreferredType can also be omitted; it is then considered to be String for dates and Number for all other values. This is how the operators + and == call ToPrimitive().

> var empty = {};
> empty.valueOf() === empty
true
> empty.toString()
'[object Object]'

> Number({})
NaN

> var n = { valueOf: function () { return 123 } };
> Number(n)
123
> String(n)
'[object Object]'

> var s = { toString: function () { return '7'; } };
> String(s)
'7'
> Number(s)
7