How to type in javascript

JavaScript Data Types

To be able to operate on variables, it is important to know something about the type.

Without data types, a computer cannot safely solve this:

Does it make any sense to add «Volvo» to sixteen? Will it produce an error or will it produce a result?

JavaScript will treat the example above as:

Note

When adding a number and a string, JavaScript will treat the number as a string.

Example

Example

JavaScript evaluates expressions from left to right. Different sequences can produce different results:

JavaScript:

JavaScript:

In the first example, JavaScript treats 16 and 4 as numbers, until it reaches «Volvo».

In the second example, since the first operand is a string, all operands are treated as strings.

JavaScript Types are Dynamic

JavaScript has dynamic types. This means that the same variable can be used to hold different data types:

Example

JavaScript Strings

A string (or a text string) is a series of characters like «John Doe».

Strings are written with quotes. You can use single or double quotes:

Example

// Using double quotes:
let carName1 = «Volvo XC60»;

// Using single quotes:
let carName2 = ‘Volvo XC60’;

You can use quotes inside a string, as long as they don’t match the quotes surrounding the string:

Example

// Single quote inside double quotes:
let answer1 = «It’s alright»;

// Single quotes inside double quotes:
let answer2 = «He is called ‘Johnny'»;

// Double quotes inside single quotes:
let answer3 = ‘He is called «Johnny»‘;

You will learn more about strings later in this tutorial.

JavaScript Numbers

All JavaScript numbers are stored as decimal numbers (floating point).

Numbers can be written with, or without decimals:

Example

// With decimals:
let x1 = 34.00;

// Without decimals:
let x2 = 34;

Exponential Notation

Extra large or extra small numbers can be written with scientific (exponential) notation:

Example

Note

Most programming languages have many number types:

Whole numbers (integers):
byte (8-bit), short (16-bit), int (32-bit), long (64-bit)

Real numbers (floating-point):
float (32-bit), double (64-bit).

Javascript numbers are always one type:
double (64-bit floating point).

You will learn more about numbers later in this tutorial.

JavaScript BigInt

All JavaScript numbers are stored in a a 64-bit floating-point format.

JavaScript BigInt is a new datatype (ES2020) that can be used to store integer values that are too big to be represented by a normal JavaScript Number.

Example

You will learn more about BigInt later in this tutorial.

JavaScript Booleans

Booleans can only have two values: true or false .

Example

Booleans are often used in conditional testing.

You will learn more about booleans later in this tutorial.

JavaScript Arrays

JavaScript arrays are written with square brackets.

Array items are separated by commas.

The following code declares (creates) an array called cars , containing three items (car names):

Example

Array indexes are zero-based, which means the first item is [0], second is [1], and so on.

You will learn more about arrays later in this tutorial.

JavaScript Objects

JavaScript objects are written with curly braces <> .

Object properties are written as name:value pairs, separated by commas.

Example

The object (person) in the example above has 4 properties: firstName, lastName, age, and eyeColor.

You will learn more about objects later in this tutorial.

The typeof Operator

You can use the JavaScript typeof operator to find the type of a JavaScript variable.

The typeof operator returns the type of a variable or an expression:

Example

typeof «» // Returns «string»
typeof «John» // Returns «string»
typeof «John Doe» // Returns «string»

Example

typeof 0 // Returns «number»
typeof 314 // Returns «number»
typeof 3.14 // Returns «number»
typeof (3) // Returns «number»
typeof (3 + 4) // Returns «number»

You will learn more about typeof later in this tutorial.

Undefined

In JavaScript, a variable without a value, has the value undefined . The type is also undefined .

Example

Any variable can be emptied, by setting the value to undefined . The type will also be undefined .

Example

Empty Values

An empty value has nothing to do with undefined .

An empty string has both a legal value and a type.

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