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

These will convert:

Number('3.14');
Number(Math.PI);
Number(' ');
Number('');

These will not convert:

Number('99 88');
Number('John');

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

Converting Dates to Numbers

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

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

The date method getTime() does the same.

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

Converting Dates to Strings

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

String(Date()); // returns "Thu Jul 17 2014 15:38:19 GMT+0200 (W. Europe Daylight Time)"

The Date method toString() does the same.

Example

Date().toString(); // returns "Thu Jul 17 2014 15:38:19 GMT+0200 (W. Europe Daylight Time)"

Converting Booleans to Numbers

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

Number(false); // returns 0
Number(true); // returns 1

Converting Booleans to Strings

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

String(false); // returns "false"
String(true); // returns "true"

The Boolean method toString() does the same.

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:

  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:

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

// if myVar = \{name:"John"\}  // 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:

// 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.