JavaScript Type Conversion

JavaScript allows variables to be converted from one data type to another. This can be done by:

• Explicitly using JavaScript functions
• Implicitly by JavaScript itself

Types of 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

Converting Strings to Numbers

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

• Numeric strings (e.g., "3.14") convert to numbers (3.14).
• An empty string ("") converts to 0.
• A non-numeric string ("John") converts to NaN (Not a Number).

Example

console.log(Number("3.14"));    // 3.14
console.log(Number(Math.PI));   // 3.141592653589793
console.log(Number(" "));      // 0
console.log(Number(""));       // 0
console.log(Number("99 88"));  // NaN
console.log(Number("John"));   // NaN

The Unary + Operator

The unary + operator can also be used to convert a string into a number.

let y = "5";      // y is a string
let x = +y;       // x is a number
console.log(x);   // 5

If the string cannot be converted, it results in NaN

let y = "John";
let x = +y;
console.log(x);  // NaN

Converting Numbers to Strings

The String() function converts numbers to strings.

console.log(String(123));       // "123"
console.log(String(100 + 23));  // "123"

The toString() method does the same:

let x = 123;
console.log(x.toString());      // "123"
console.log((100 + 23).toString()); // "123"

Converting Dates to Numbers

Use the Number() function to convert a date to milliseconds since January 1, 1970.

let d = new Date();
console.log(Number(d));  // e.g., 1678945200000

The getTime() method does the same:

console.log(d.getTime()); // e.g., 1678945200000

Converting Dates to Strings

Use the String() function to convert a date to a string.

console.log(String(new Date())); // e.g, Thu Mar 20 2025 15:48:20 GMT+0500

The toString() method also works:

console.log(new Date().toString()); // e.g, Thu Mar 20 2025 15:49:53 GMT+0500

Converting Booleans to Numbers

Using Number(): false converts to 0 and true converts to 1.

console.log(Number(false)); // 0
console.log(Number(true));  // 1

Converting Booleans to Strings

Using String(): true and false convert to string representations.

console.log(String(false)); // "false"
console.log(String(true));  // "true"

The toString() method works similarly

console.log(false.toString()); // "false"
console.log(true.toString());  // "true"

Automatic Type Conversion

JavaScript automatically converts types when needed. However, results may be unexpected:

console.log(5 + null);    // 5  (null converts to 0)
console.log("5" + null);  // "5null" (null converts to "null")
console.log("5" + 2);     // "52"  (number 2 converts to "2")
console.log("5" - 2);     // 3    ("5" converts to 5)
console.log("5" * "2");   // 10   (both convert to numbers)

Automatic String Conversion

JavaScript calls toString() when a value is expected as a string:

let obj = { name: "John" };
console.log(obj.toString());  // "[object Object]"
let arr = [1, 2, 3, 4];
console.log(arr.toString());  // "1,2,3,4"

Numbers and booleans are also automatically converted:

console.log(123 + "");   // "123"
console.log(true + "");  // "true"
console.log(false + ""); // "false"