Data types

A variable in JavaScript can contain any data. A variable can at one moment be a string and later recieve a numeric value:

// no error
let message = "hello";
message = 123456;

Programming languages that allow such thing are called “dynamically typed”, meaning that there are data types, but variables are not bound to any of them.

There are 7 basic data types in JavaScript. Here we’ll study the basics, and in next chapters we’ll talk about each of them in detail.

A number

let n = 123;
n = 12.345;

The number type serves both for integer and floating point numbers.

There are many operations for numbers, e.g. multiplication *, division /, addition +, substraction - and so on.

Besides regular numbers, there are so-called “special numeric values” which also belong to that type: Infinity, -Infinity and NaN.

  • Infinity represents the mathematical Infinity ∞. It is a special value that’s greater than any number.

    We can get it as a result of division by zero:

    alert( 1 / 0 ); // Infinity

    Or just mention it in the code directly:

    alert( Infinity ); // Infinity
  • NaN represents a computational error. It is a result of an incorrect or an undefined mathematical operation, for instance:

    alert( "not a number" / 2 ); // NaN, such division is erroneous

    NaN is sticky. Any further operation on NaN would give NaN:

    alert( "not a number" / 2 + 5 ); // NaN

    So, if there’s NaN somewhere in a mathematical expression, it propagates to the whole result.

Mathematical operations are safe

Doing maths is safe in JavaScript. We can do anything: divide by zero, treat non-numeric strings as numbers, etc.

The script will never stop with a fatal error (“die”). At worst we’ll get NaN as the result.

Special numeric values formally belong to the “number” type. Of course they are not numbers in a common sense of this word.

We’ll see more into working with numbers in the chapter Numbers.

A string

A string in JavaScript must be quoted.

let str = "Hello";
let str2 = 'Single quotes are ok too';
let phrase = `can embed ${str}`;

In JavaScript, there are 3 types of quotes.

  1. Double quotes: "Hello".
  2. Single quotes: 'Hello'.
  3. Backticks: `Hello`.

Double and single quotes are “simple” quotes. There’s no difference between them in JavaScript.

Backticks are “extended functionality” quotes. They allow to embed variables and expressions into a string by wrapping them in ${…}, for example:

let name = "John";

// embed a variable
alert( `Hello, ${name}!` ); // Hello, John!

// embed an expression
alert( `the result is ${1 + 2}` ); // the result is 3

The expression inside ${…} is evaluated and the result becomes a part of the string. We can put anything there: a variable like name or an arithmetical expression like 1 + 2 or something more complex.

Please note that this only can be done in backticks, other quotes do not allow such embedding!

alert( "the result is ${1 + 2}" ); // the result is ${1 + 2} (double quotes do nothing)

We’ll cover strings more thoroughly in the chapter Strings.

There is no character type.

In some languages, there is a special “character” type for a single character. For example, in the C language and in Java it is char.

In JavaScript, there is no such type. There’s only one type: string. A string may consist of only one character or many of them.

A boolean (logical type)

The boolean type has only two values: true and false.

This type is commonly used to store yes/no values: true means “yes, correct”, and false means the “no, incorrect”.

For instance:

let nameFieldChecked = true; // yes, name field is checked
let ageFieldChecked = false; // no, age field is not checked

Boolean values also come as a result of comparisons:

let isGreater = 4 > 1;

alert( isGreater ); // true (the comparison result is "yes")

We’ll cover booleans more deeply later in the chapter Logical operators.

The “null” value

The special null value does not belong to any type of those described above.

It forms a separate type of its own, which contains only the null value:

let age = null;

In JavaScript null is not a “reference to a non-existing object” or a “null pointer” like in some other languages.

It’s just a special value which has the sense of “nothing”, “empty” or “value unknown”.

The code above states that the age is unknown or empty for some reason.

The “undefined” value

The special value undefined stands apart. It makes a type of its own, just like null.

The meaning of undefined is “value is not assigned”.

If a variable is declared, but not assigned, then its value is exactly undefined:

let x;

alert(x); // shows "undefined"

Technically, it is possible to assign any variable to undefined:

let x = 123;

x = undefined;

alert(x); // "undefined"

…But it’s not recommended to do that. Normally, we use null to write an “empty” or an “unknown” value into the variable, and undefined is only used for checks, to see if the variable is assigned or similar.

Objects and Symbols

The object type is special.

All other types are called “primitive”, because their values can contain only a single thing (be it a string or a number or whatever). In contrast, objects are used to store collections data and more complex entities. We’ll deal with them later in the chapter Objects after we know enough about primitives.

The symbol type is used to create unique identifiers for objects. We have to mention it here for completeness, but it’s better to study them after objects.

The typeof operator

The typeof operator returns the type of the argument. It’s useful when we want to process values of different types differently, or just want to make a quick check.

It supports two forms of syntax:

  1. As an operator: typeof x.
  2. Function style: typeof(x).

In other words, it works both with the brackets or without them. The result is the same.

The call to typeof x returns a string with the type name:

typeof undefined // "undefined"

typeof 0 // "number"

typeof true // "boolean"

typeof "foo" // "string"

typeof Symbol("id") // "symbol"

typeof Math // "object"  (1)

typeof null // "object"  (2)

typeof alert // "function"  (3)

The last three lines may need additional explanations:

  1. Math is a built-in object that provides mathematical operations. We will learn it in the chapter Numbers. Here it serves just as an example of an object.
  2. The result of typeof null is "object". That’s wrong. It is an officially recognized error in typeof, kept for compatibility. Of course, null is not an object. It is a special value with a separate type of its own. So, again, that’s an error in the language.
  3. The result of typeof alert is "function", because alert is a function of the language. We’ll study functions in the next chapters, and we’ll see that there’s no special “function” type in the language. Functions belong to the object type. But typeof treats them differently. Formally, it’s incorrect, but very convenient in practice.


There are 7 basic types in JavaScript.

  • number for numbers of any kind: integer or floating-point.
  • string for strings. A string may have one more more characters, there’s no separate single-character type.
  • boolean for true/false.
  • null for unknown values – a standalone type that has a single value null.
  • undefined for unassigned values – a standalone type that has a single value undefined.
  • object for more complex data structures.
  • symbol for unique identifiers.

The typeof operator allows to see which type is stored in the variable.

  • Two forms: typeof x or typeof(x).
  • Returns a string with the name of the type, like "string".
  • For null returns "object" – that’s the error in the language, it’s not an object in fact.

In the next chapters we’ll concentrate on primitive values and once we’re familiar with that, then we’ll move on to objects.


importance: 5

What is the output of the script?

let name = "Ilya";

alert( `hello ${1}` ); // ?

alert( `hello ${"name"}` ); // ?

alert( `hello ${name}` ); // ?

Backticks embed the expression inside ${...} into the string.

let name = "Ilya";

// the expression is a number 1
alert( `hello ${1}` ); // Hello, 1

// the expression is a variable, embed it
alert( `hello ${name}` ); // Hello, Ilya

// the expression is a string "name"
alert( `hello ${"name"}` ); // Hello, name
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