JavaScript variables can hold different data types: numbers, strings, objects and more:let length = 16; // Number
let lastName = “Johnson”; // String
let x = {firstName:”John”, lastName:”Doe”}; // Object
The Concept of Data Types
In programming, data types is an important concept.
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:let x = 16 + “Volvo”;
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:let x = “16” + “Volvo”;
When adding a number and a string, JavaScript will treat the number as a string.
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript</h2>
<p>When adding a number and a string, JavaScript will treat the number as a string.</p>
<p id="demo"></p>
<script>
let x = 16 + "Volvo";
document.getElementById("demo").innerHTML = x;
</script>
</body>
</html>
Result:
JavaScript
When adding a number and a string, JavaScript will treat the number as a string.
16Volvo
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript</h2>
<p>When adding a string and a number, JavaScript will treat the number as a string.</p>
<p id="demo"></p>
<script>
let x = "Volvo" + 16;
document.getElementById("demo").innerHTML = x;
</script>
</body>
</html>
Result:
JavaScript
When adding a string and a number, JavaScript will treat the number as a string.
Volvo16
JavaScript evaluates expressions from left to right. Different sequences can produce different results:
JavaScript:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript</h2>
<p>JavaScript evaluates expressions from left to right. Different sequences can produce different results:</p>
<p id="demo"></p>
<script>
let x = 16 + 4 + "Volvo";
document.getElementById("demo").innerHTML = x;
</script>
</body>
</html>
Result:
JavaScript
JavaScript evaluates expressions from left to right. Different sequences can produce different results:
20Volvo
JavaScript:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript</h2>
<p>JavaScript evaluates expressions from left to right. Different sequences can produce different results:</p>
<p id="demo"></p>
<script>
let x = "Volvo" + 16 + 4;
document.getElementById("demo").innerHTML = x;
</script>
</body>
</html>
Result:
JavaScript
JavaScript evaluates expressions from left to right. Different sequences can produce different results:
Volvo164
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Data Types</h2>
<p>JavaScript has dynamic types. This means that the same variable can be used to hold different data types:</p>
<p id="demo"></p>
<script>
let x; // Now x is undefined
x = 5; // Now x is a Number
x = "John"; // Now x is a String
document.getElementById("demo").innerHTML = x;
</script>
</body>
</html>
Result:
JavaScript Data Types
JavaScript has dynamic types. This means that the same variable can be used to hold different data types:
John
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Strings</h2>
<p>Strings are written with quotes. You can use single or double quotes:</p><p id="demo"></p>
<script>
let carName1 = "Volvo XC60";
let carName2 = 'Volvo XC60';
document.getElementById("demo").innerHTML =
carName1 + "<br>" +
carName2;
</script>
</body>
</html>
Result:
JavaScript Strings
Strings are written with quotes. You can use single or double quotes:
Volvo XC60
Volvo XC60
You can use quotes inside a string, as long as they don’t match the quotes surrounding the string:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Strings</h2>
<p>You can use quotes inside a string, as long as they don't match the quotes surrounding the string:</p>
<p id="demo"></p>
<script>
let answer1 = "It's alright";
let answer2 = "He is called 'Johnny'";
let answer3 = 'He is called "Johnny"';
document.getElementById("demo").innerHTML =
answer1 + "<br>" +
answer2 + "<br>" +
answer3;
</script>
</body>
</html>
Result:
JavaScript Strings
You can use quotes inside a string, as long as they don’t match the quotes surrounding the string:
It’s alright
He is called ‘Johnny’
He is called “Johnny”
You will learn more about strings later in this tutorial.
JavaScript Numbers
JavaScript has only one type of numbers.
Numbers can be written with, or without decimals:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Numbers</h2>
<p>Numbers can be written with, or without decimals:</p>
<p id="demo"></p>
<script>
let x1 = 34.00;
let x2 = 34;
let x3 = 3.14;
document.getElementById("demo").innerHTML =
x1 + "<br>" + x2 + "<br>" + x3;
</script>
</body>
</html>
Result:
JavaScript Numbers
Numbers can be written with, or without decimals:
34
34
3.14
Extra large or extra small numbers can be written with scientific (exponential) notation:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Numbers</h2>
<p>Extra large or extra small numbers can be written with scientific (exponential) notation:</p>
<p id="demo"></p>
<script>
let y = 123e5;
let z = 123e-5;
document.getElementById("demo").innerHTML =
y + "<br>" + z;
</script>
</body>
</html>
Result:
JavaScript Numbers
Extra large or extra small numbers can be written with scientific (exponential) notation:
12300000
0.00123
You will learn more about numbers later in this tutorial.
JavaScript Booleans
Booleans can only have two values: true or false.
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Booleans</h2>
<p>Booleans can have two values: true or false:</p>
<p id="demo"></p>
<script>
let x = 5;
let y = 5;
let z = 6;
document.getElementById("demo").innerHTML =
(x == y) + "<br>" + (x == z);
</script>
</body>
</html>
Result:
JavaScript Booleans
Booleans can have two values: true or false:
true
false
Booleans are often used in conditional testing.
You will learn more about conditional testing 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
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Array indexes are zero-based, which means the first item is [0].</p>
<p id="demo"></p>
<script>
const cars = ["Saab","Volvo","BMW"];
document.getElementById("demo").innerHTML = cars[0];
</script>
</body>
</html>
Result:
JavaScript Arrays
Array indexes are zero-based, which means the first item is [0].
Saab
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Objects</h2>
<p id="demo"></p>
<script>
const person = {
firstName : "John",
lastName : "Doe",
age : 50,
eyeColor : "blue"
};
document.getElementById("demo").innerHTML =
person.firstName + " is " + person.age + " years old.";
</script>
</body>
</html>
Result:
JavaScript Objects
John is 50 years old.
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript typeof</h2>
<p>The typeof operator returns the type of a variable or an expression.</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML =
typeof "" + "<br>" +
typeof "John" + "<br>" +
typeof "John Doe";
</script>
</body>
</html>
Result:
JavaScript typeof
The typeof operator returns the type of a variable or an expression.
string
string
string
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript typeof</h2>
<p>The typeof operator returns the type of a variable or an expression.</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML =
typeof 0 + "<br>" +
typeof 314 + "<br>" +
typeof 3.14 + "<br>" +
typeof (3) + "<br>" +
typeof (3 + 4);
</script>
</body>
</html>
Result:
JavaScript typeof
The typeof operator returns the type of a variable or an expression.
number
number
number
number
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript</h2>
<p>The value (and the data type) of a variable with no value is <b>undefined</b>.</p>
<p id="demo"></p>
<script>
let car;
document.getElementById("demo").innerHTML =
car + "<br>" + typeof car;
</script>
</body>
</html>
Result:
JavaScript
The value (and the data type) of a variable with no value is undefined.
undefined
undefined
Any variable can be emptied, by setting the value to undefined. The type will also be undefined.
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript</h2>
<p>Variables can be emptied if you set the value to <b>undefined</b>.</p>
<p id="demo"></p>
<script>
let car = "Volvo";
car = undefined;
document.getElementById("demo").innerHTML = car + "<br>" + typeof car;
</script>
</body>
</html>
Result:
JavaScript
Variables can be emptied if you set the value to undefined.
undefined
undefined
Empty Values
An empty value has nothing to do with undefined.
An empty string has both a legal value and a type.
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript</h2>
<p>An empty string has both a legal value and a type:</p>
<p id="demo"></p>
<script>
let car = "";
document.getElementById("demo").innerHTML =
"The value is: " +
car + "<br>" +
"The type is: " + typeof car;
</script>
</body>
</html>
Result:
JavaScript
An empty string has both a legal value and a type:
The value is:
The type is: string