Values and Data Types

Programs may be thought of as being made up of two things:

  1. Data
  2. Operations that manipulate data

This chapter focuses primarily on the first of these two fundamental components, data.

Data can be stored in a program in a variety of ways. The most basic unit of data is a value.

A value is a specific piece of data, such as a word or a number. Some examples are 5, 5.2, and "Hello, World!".

Each value belongs to a category called a data type. We will see many different data types throughout the course, the first two of which are the number and string types. Numeric values such as 4 and 3.3 are numbers. Sequences of characters enclosed in quotes, such as "Hello, World!", are strings, so-called because they contain a string of letters. Strings must be enclosed in either single or double quotes.

If you are not sure what data type a value falls into, precede the value with typeof.

console.log(typeof "Hello, World!");
console.log(typeof 17);
console.log(typeof 3.14);

Console Output


Not surprisingly, JavaScript reports that the data type of "Hello, World!" is string, while the data type of both 17 and 3.14 is number. Note that some JavaScript environments may print type names and strings with single quotes around them, as in 'string' 'number', and 'hello'.


Notice that console.log(typeof "Hello, World!"); prints out string to the console. The typeof keyword is not printed to the console because the statement typeof "Hello, World!" is an expression. Briefly, expressions are code segments that are reduced to a value. We will learn more about expressions soon.

We say that an expression returns a value. That is, typeof "Hello, World!" returns the value string.


typeof is a JavaScript entity known as an operator. It is similar to a function in that it carries out some kind of action, though the syntax is different from that of functions (notice using typeof does not require parentheses).

We will more about operators in upcoming sections.

There are data types other than string and number, including object and function, which we will learn about in future chapters.

More On Strings

What about values like "17" and "3.2"? They look like numbers, but they are in quotation marks like strings.

Run the following code to find out.

Try it!
console.log(typeof "17");
console.log(typeof "3.2");

What is the data type of the values "17" and "3.2"?

Strings in JavaScript can be enclosed in either single quotes (') or double quotes (").

console.log(typeof 'This is a string');
console.log(typeof "And so is this");

Console Output


Double-quoted strings can contain single quotes inside them, as in "Bruce's beard", and single quoted strings can have double quotes inside them, as in 'The knights who say "Ni!"'.

JavaScript doesn’t care whether you use single or double quotes to surround your strings. Once it has parsed the text of your program or command, the way it stores the value is identical in all cases, and the surrounding quotes are not part of the value.


If a string contains a single quote (such as "Bruce's beard") then surrounding it with single quotes gives unexpected results.

What happens if you run the following piece of code?

console.log("Bruce's beard");

More On Numbers

When you type a large integer value, you might be tempted to use commas between groups of three digits, as in 42,000. This is not a legal integer in JavaScript, but it does mean something else, which is legal:


Console Output

42 0

Well, that’s not what we expected at all! Because of the comma, JavaScript chose to treat 42,000 as a pair of values. In fact, the console.log function can print any number of values as long as you separate them by commas. Notice that the values are separated by spaces when they are displayed.

console.log(42, 17, 56, 34, 11, 4.35, 32);
console.log(3.4, "hello", 45);

Console Output

42 17 56 34 11 4.35 32
3.4 hello 45

Remember not to put commas or spaces in your integers, no matter how big they are. Also revisit what we said in the chapter How Programs Work formal languages are strict, the notation is concise, and even the smallest change might mean something quite different from what you intend.

Type Systems

Every programming language has a type system, which is the set of rules that determine how the languages deals with data of different types. In particular, how values are divided up into different data types is one characteristic of a type system.

In many programming languages, integers and floats are considered to be different data types. For example, in Python 42 is of the int data type, while 42.0 is of the float data type.


While JavaScript does not distinguish between floats and integers, at times we may wish to do so in our programs. For example, an inventory-tracking program stores items and the number of each number in stock. Since a store cannot have 3.5 shirts in stock, the programmer makes the quantity of each item integer values as opposed to floats.

When discussing the differences between programming languages, the details of type systems are one of the main factors that programmers consider. There are other aspects of type systems beyond just how values are categorized. We will explore these in future lessons.

Check Your Understanding


Which of these is not a data type in JavaScript?

  1. number
  2. string
  3. letter
  4. object