In programming, type conversion is the process of converting data of one type to another. For example: converting String data to Number.

There are two types of type conversion in JavaScript.

• Implicit Conversion – automatic type conversion
• Explicit Conversion – manual type conversion

JavaScript Implicit Conversion

In certain situations, JavaScript automatically converts one data type to another (to the right type). This is known as implicit conversion.

Example 1: Implicit Conversion to String

// numeric string used with + gives string type
let result;

result = '3' + 2; 
console.log(result) // "32"

result = '3' + true; 
console.log(result); // "3true"

result = '3' + undefined; 
console.log(result); // "3undefined"

result = '3' + null; 
console.log(result); // "3null"

Note: When a number is added to a string, JavaScript converts the number to a string before concatenation.

Example 2: Implicit Conversion to Number

// numeric string used with - , / , * results number type

let result;

result = '4' - '2'; 
console.log(result); // 2

result = '4' - 2;
console.log(result); // 2

result = '4' * 2;
console.log(result); // 8

result = '4' / 2;
console.log(result); // 2

Example 3: Non-numeric String Results to NaN

// non-numeric string used with - , / , * results to NaN

let result;

result = 'hello' - 'world';
console.log(result); // NaN

result = '4' - 'hello';
console.log(result); // NaN

Example 4: Implicit Boolean Conversion to Number

// if boolean is used, true is 1, false is 0

let result;

result = '4' - true;
console.log(result); // 3

result = 4 + true;
console.log(result); // 5

result = 4 + false;
console.log(result); // 4

Note: JavaScript considers 0 as false and all non-zero number as true. And, if true is converted to a number, the result is always 1.

Example 5: null Conversion to Number

// null is 0 when used with number
let result;

result = 4 + null;
console.log(result);  // 4

result = 4 - null;
console.log(result);  // 4

Example 6: undefined used with number, boolean or null

// Arithmetic operation of undefined with number, boolean or null gives NaN

let result;

result = 4 + undefined;
console.log(result);  // NaN

result = 4 - undefined;
console.log(result);  // NaN

result = true + undefined;
console.log(result);  // NaN

result = null + undefined;
console.log(result);  // NaN

JavaScript Explicit Conversion

You can also convert one data type to another as per your needs. The type conversion that you do manually is known as explicit type conversion.

In JavaScript, explicit type conversions are done using built-in methods.

Here are some common methods of explicit conversions.

1. Convert to Number Explicitly

To convert numeric strings and boolean values to numbers, you can use Number(). For example,

let result;

// string to number
result = Number('324');
console.log(result); // 324

result = Number('324e-1')  
console.log(result); // 32.4

// boolean to number
result = Number(true);
console.log(result); // 1

result = Number(false);
console.log(result); // 0

In JavaScript, empty strings and null values return 0. For example,

let result;
result = Number(null);
console.log(result);  // 0

let result = Number(' ')
console.log(result);  // 0

If a string is an invalid number, the result will be NaN. For example,

let result;
result = Number('hello');
console.log(result); // NaN

result = Number(undefined);
console.log(result); // NaN

result = Number(NaN);
console.log(result); // NaN

let result;
result = parseInt('20.01');
console.log(result); // 20

result = parseFloat('20.01');
console.log(result); // 20.01

result = +'20.01';
console.log(result); // 20.01

result = Math.floor('20.01');
console.log(result); // 20

2. Convert to String Explicitly

To convert other data types to strings, you can use either String() or toString(). For example,

//number to string
let result;
result = String(324);
console.log(result);  // "324"

result = String(2 + 4);
console.log(result); // "6"

//other data types to string
result = String(null);
console.log(result); // "null"

result = String(undefined);
console.log(result); // "undefined"

result = String(NaN);
console.log(result); // "NaN"

result = String(true);
console.log(result); // "true"

result = String(false);
console.log(result); // "false"

// using toString()
result = (324).toString();
console.log(result); // "324"

result = true.toString();
console.log(result); // "true"

Note: String() takes null and undefined and converts them to string. However, toString() gives error when null are passed.

3. Convert to Boolean Explicitly

To convert other data types to a boolean, you can use Boolean().

In JavaScript, undefined, null, 0, NaN, '' converts to false. For example,

let result;
result = Boolean('');
console.log(result); // false

result = Boolean(0);
console.log(result); // false

result = Boolean(undefined);
console.log(result); // false

result = Boolean(null);
console.log(result); // false

result = Boolean(NaN);
console.log(result); // false

All other values give true. For example,

result = Boolean(324);
console.log(result); // true

result = Boolean('hello');
console.log(result); // true

result = Boolean(' ');
console.log(result); // true

JavaScript Type Conversion Table

The table shows the conversion of different values to String, Number, and Boolean in JavaScript.

You will learn about the conversion of objects and arrays to other data types in later tutorials.

Related Examples:

• JavaScript Dates to Numbers
• JavaScript Dates to Strings