Javascript Data Types

Question 1

JavaScript Data Types
JavaScript variables can hold many data types: numbers, strings, objects and more:

var length = 16;                               // Number
var lastName = "Johnson";                      // String
var x = {firstName:"John", lastName:"Doe"};    // Object

In programming, data types are 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:

var x = 16 + "Volvo";
Javascript reads the above as: var x = "16" + "Volvo";.
When adding a number and a string, JavaScript will treat the number as a string.

What code will produce “16Volvo” in the display?
How about “Volvo16” in the display?
Now if x is displayed as “20Volvo” with values of 16, 4, and Volvo?
How about “Volvo164” with values of Volvo, 16, and 4?

Answer 1

var x = 16 + "Volvo";
document.getElementById("demo").innerHTML = x;

var x = “Volvo” + 16;

var x = 16 + 4 + "Volvo";
document.getElementById("demo").innerHTML = x;

var x = "Volvo" + 16 + 4;
document.getElementById("demo").innerHTML = x;

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.

Question 2

JavaScript Types are Dynamic
JavaScript has dynamic types. This means that the same variable can be used to hold different data types.
For example:
//Now x is undefined
//Now x is a Number
//Now x is a String

Answer 2

var x; // Now x is undefined
x = 5; // Now x is a Number
x = “John”; // Now x is a String

Question 3

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: Volvo XC60 in single and double quotes with two different var names with a display of:

Volvo XC60
Volvo XC60

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

// Double quote string
// Single quotes inside double quotes string
// Double quotes inside single quotes string

Answer 3

var carName1 = "Volvo XC60";
var carName2 = 'Volvo XC60';

document.getElementById(“demo”).innerHTML =
carName1 + “br>” +
carName2;

var answer1 = "It's alright";             // Single quote inside double quotes
var answer2 = "He is called 'Johnny'";    // Single quotes inside double quotes
var answer3 = 'He is called "Johnny"';    // Double quotes inside single quotes

Question 4

JavaScript Numbers
JavaScript has only one type of numbers.

Numbers can be written with, or without decimals:

var // Written with decimals
var // Written without decimals
Displays:
34
34
3.14

Extra large or extra small numbers can be written with scientific (exponential) notation:
// 12300000
// 0.00123

Answer 4

var x1 = 34.00;
var x2 = 34;
var x3 = 3.14;

document.getElementById(“demo”).innerHTML =
x1 + “br>” + x2 + “br>” + x3;

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

var y = 123e5;      // 12300000
var z = 123e-5;     // 0.00123

Question 5

JavaScript Booleans
Booleans can only have two values: true or false.

var x = 5;
var y = 5;
var z = 6;
(x == y)       // Returns true
(x == z)       // Returns false

What does the full code look like?

Booleans are often used in conditional testing.

Answer 5

script>
var x = 5;
var y = 5;
var z = 6;
document.getElementById("demo").innerHTML =
(x == y) + "br>" + (x == z);
/script>

Booleans are often used in conditional testing.

Question 6

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):

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

Answer 6

var cars = [“Saab”,”Volvo”,”BMW”];

document.getElementById(“demo”).innerHTML = cars[0];

Question 7

JavaScript Objects
JavaScript objects are written with curly braces {}.

Object properties are written as name:value pairs, separated by commas.
The object (person) in the example above has 4 properties: firstName, lastName, age, and eyeColor.

var person = {firstName:”John”, lastName:”Doe”, age:50, eyeColor:”blue”};

What would code look like displaying: John is 50 years old.

Answer 7

var person = {
  firstName : "John",
  lastName  : "Doe",
  age     : 50,
  eyeColor  : "blue"
};

document. getElementById(“demo”).innerHTML =
person. firstName + “ is “ + person.age + “ years old.”;

Question 8

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: What would code look like that returns 3 strings like this one on top of the other?

”” // Returns “string”
“John” // Returns “string”
“John Doe” // Returns “string”

Then what it looks like the typeof for these numbers?

0              // Returns "number"
314            // Returns "number"
3.14           // Returns "number"
(3)            // Returns "number"
(3 + 4)        // Returns "number"

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

Answer 8

document.getElementById(“demo”).innerHTML =
typeof “” + “br>” +
typeof “John” + “br>” +
typeof “John Doe”;

string
string
string

document.getElementById("demo").innerHTML = 
typeof 0 + "br>" + 
typeof 314 + "br>" +
typeof 3.14 + "br>" +
typeof (3) + "br>" +
typeof (3 + 4);

number
number
number
number
number

Question 9

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

var car; // Value is undefined, type is undefined

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

car = undefined; // Value is undefined, type is undefined

Answer 9

var car;
document.getElementById(“demo”).innerHTML =
car + “br>” + typeof car;

displays:
The value (and the data type) of a variable with no value is undefined.

undefined
undefined

var car = "Volvo";
car = undefined;
document.getElementById("demo").innerHTML =
car + "br>" + typeof car;

Variables can be emptied if you set the value to undefined.

undefined
undefined

Question 10

Empty Values
An empty value has nothing to do with undefined.

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

var car = “”; // The value is “”, the typeof is “string”

Answer 10

var car = "";
document.getElementById("demo").innerHTML =
"The value is: " +
car + "br>" +
"The type is: " + typeof car;

displays:
An empty string has both a legal value and a type:

The value is:
The type is: string

Question 11

Null
In JavaScript null is “nothing”. It is supposed to be something that doesn’t exist.

Unfortunately, in JavaScript, the data type of null is an object.

You can consider it a bug in JavaScript that typeof null is an object. It should be null.

You can empty an object by setting it to null:

var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
person = null;    // Now value is null, but type is still an object

Then

You can also empty an object by setting it to undefined:

var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
person = undefined;   // Now both value and type is undefined

Answer 11

var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
person = null;
document.getElementById("demo").innerHTML = typeof person;

displays:
Objects can be emptied by setting the value to null.

object

then

var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
person = undefined;
document.getElementById("demo").innerHTML = person;

displays:
Objects can be emptied by setting the value to undefined.

undefined

Question 12

Difference Between Undefined and Null
undefined and null are equal in value but different in type:

typeof undefined // undefined
typeof null // object

null === undefined // false
null == undefined // true

Answer 12

document.getElementById("demo").innerHTML =
typeof undefined + "br>" +
typeof null + "br>br>" +
(null === undefined) + "br>" +
(null == undefined);

displays:
Undefined and null are equal in value but different in type:

undefined
object

false
true

Question 13

Primitive Data
A primitive data value is a single simple data value with no additional properties and methods.

The typeof operator can return one of these primitive types:

string
number
boolean
undefined

typeof “John” // Returns “string”
typeof 3.14 // Returns “number”
typeof true // Returns “boolean”
typeof false // Returns “boolean”
typeof x // Returns “undefined” (if x has no value)

Answer 13

document.getElementById("demo").innerHTML = 
typeof "john" + "br>" + 
typeof 3.14 + "br>" +
typeof true + "br>" +
typeof false + "br>" +
typeof x;

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

string
number
boolean
boolean
undefined

Question 14

Complex Data
The typeof operator can return one of two complex types:

function
object
The typeof operator returns object for both objects, arrays, and null.

The typeof operator does not return object for functions.

typeof {name:’John’, age:34} // Returns “object”
typeof [1,2,3,4] // Returns “object” (not “array”, see note below)
typeof null // Returns “object”
typeof function myFunc(){} // Returns “function”

The typeof operator returns “object” for arrays because in JavaScript arrays are objects.

Answer 14

document.getElementById("demo").innerHTML = 
typeof {name:'john', age:34} + "br>" +
typeof [1,2,3,4] + "br>" +
typeof null + "br>" +
typeof function myFunc(){};

displays:
The typeof operator returns object for both objects, arrays, and null.

The typeof operator does not return object for functions.

object
object
object
function