JavaScript Type Conversion

• Converting Strings to Numbers
• Converting Numbers to Strings
• Converting Dates to Numbers
• Converting Numbers to Dates
• Converting Booleans to Numbers
• Converting Numbers to Booleans

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JavaScript Type Conversion

JavaScript variables can be converted to a new variable and another data type:

• By the use of a JavaScript function
• Automatically by JavaScript itself

Converting Strings to Numbers

The global method Number() converts a variable (or a value) into a number.

A numeric string (like "3.14") converts to a number (like 3.14).

An empty string (like "") converts to 0.

A non numeric string (like "John") converts to NaN (Not a Number).

Examples

Number("3.14")

Number(Math.PI)

Number(" ")

Number("")

Number Methods

In the chapter Number Methods, you will find more methods that can be used to convert strings to numbers:

The Unary + Operator

The unary + operator can be used to convert a variable to a number:

Example

let y = "5";
// y is a string

let x = + y;
// x is a number

If the variable cannot be converted, it will still become a number, but with the value NaN (Not a Number):

Example

let y = "John";
// y is a string

let x = + y;      // x is a number (NaN)

Converting Numbers to Strings

The global method String() can convert numbers to strings.

It can be used on any type of numbers, literals, variables, or expressions:

Example

String(x)         // returns a string from a number variable x

 String(123)       // returns a string from a number literal 123

 String(100 + 23)  // returns a string from a number from an expression

The Number method toString() does the same.

Example

x.toString()
(123).toString()
(100 + 23).toString()

More Methods

In the chapter Number Methods, you will find more methods that can be used to convert numbers to strings:

Converting Dates to Numbers

The global method Number() can be used to convert dates to numbers.

Example

d = new Date();
Number(d)          // returns 1404568027739

The date method getTime() does the same.

Example

d = new Date();
d.getTime()        // returns 1404568027739

Converting Dates to Strings

The global method String() can convert dates to strings.

Example

String(Date())

The Date method toString() does the same.

Example

Date().toString()

Date Parts

Many Date Methods returns a Date part as a number.

You can use the toString() method to convert the return values to strings:

Month is Zero Based

By default, getMonth() returns an integer between 0 (January) and 11 (December).

To get a standard 1 to 12 range, you must add 1 to the result.

Example

(date.getMonth()+1).toString();

Month Name

Use the toLocaleString() method for easy formatting:

Example

date.toLocaleString('default', { month: 'long' });

Converting Booleans to Numbers

The global method Number() can also convert booleans to numbers.

Example

Number(false)     // returns 0

Number(true)      // returns 1

Converting Booleans to Strings

The global method String() can convert booleans to strings.

Example

String(false)      // returns "false"

String(true)       // returns "true"

The Boolean method toString() does the same.

Example

false.toString()   // returns "false"

  true.toString()    // returns "true"

Automatic Type Conversion

When JavaScript tries to operate on a "wrong" data type, it will try to convert the value to a "right" type.

The result is not always what you expect:

Example

5 + null    // returns 5         because null is converted to 0
"5" + null  // returns "5null"   because null is converted to "null"
"5" + 2     // returns
    "52"      because 2 is converted to "2"

 "5" - 2     // returns 3         because "5" is converted to 5
"5" * "2"   // returns
    10        because "5" and "2" are
    converted to 5 and 2

Automatic String Conversion

JavaScript automatically calls the variable's toString() function when you try to "output" an object or a variable:

Example

document.getElementById("demo").innerHTML = myVar;

// if myVar = {name:"Fjohn"}  // toString converts to "[object Object]"

// if myVar = [1,2,3,4]       // toString converts to "1,2,3,4"

// if myVar = new Date()      // toString converts to "Fri Jul 18 2014 09:08:55 GMT+0200"

Numbers and booleans are also converted, but this is not very visible:

Example

// if myVar = 123             // toString converts to "123"

// if myVar = true            // toString converts to "true"

// if myVar = false           // toString converts to "false"

JavaScript Type Conversion Table

This table shows the result of converting different JavaScript values to Number, String, and Boolean:

Values in quotes indicate string values.

Red values indicate values (some) programmers might not expect.