JavaScript: Data types

Programs work with different kinds of information: text, numbers, dates, boolean values. Every value in a program has a type.

For example:

• 'Hello, World!' is a string (string);
• 7, -198, 0, 3.14 are numbers (number);
• true and false are boolean values (boolean).

What is a data type?

A data type defines:

• which values belong to it;
• which operations can be performed with it.

For example, numbers can be added, divided, and multiplied. Strings are combined differently — using concatenation. Multiplying a string by a string makes no sense:

// Nonsense: 'mom' * 'notebook'

Numbers and strings belong to different types

An example of printing a number:

console.log(5);  // => 5
console.log(-5); // => -5

An example of printing a string:

console.log('5');  // => 5
console.log('-5'); // => -5

On the screen the result looks the same, but inside the program these are completely different things:

Numbers in JavaScript

In many languages, integers and fractional numbers are different types (for example, in Python they are int and float). JavaScript has no such division: both integers and fractional numbers belong to a single type — number.

console.log(10.234);     // => 10.234
console.log(-0.4);       // => -0.4

console.log(3.5 + 1.2);  // => 4.7
console.log(5 / 2);      // => 2.5
console.log(2.75 - 0.5); // => 2.25

Primitive types

Types like string, number, boolean are called primitive — they are built directly into the language.

JavaScript primitive types
├── number    : numbers (integer and fractional) (7, -3, 3.14)
├── string    : strings                          ('hello')
├── boolean   : boolean type                      (true, false)
├── null       : intentional absence of a value
└── undefined : value is not set

In addition to strings and numbers, JavaScript has the boolean type boolean with the values true and false, as well as the special values null and undefined. We will encounter them in more detail in the future.

How to find out the type of a value

The typeof operator returns the type as a string:

console.log(typeof 42);        // => 'number'
console.log(typeof 'hello');   // => 'string'
console.log(typeof true);      // => 'boolean'
console.log(typeof undefined); // => 'undefined'
console.log(typeof null);      // => 'object'  (a historical JS bug)

There are also composite types — arrays, objects, and others. We will get acquainted with them later. Moreover, in JavaScript you can create your own types (for example, classes), but to start with it is important to understand primitives well.

The current year is already defined in the program:

const currentYear = 2026;

Write a program that prints profile data in the following format:

Name: Anna
Birth year: 1994
Age: 32
Rating: 4.7

To do this:

• create a variable with the birth year;
• calculate the age based on the current year;
• print the data on the screen with labels on the left.