JavaScript casting: Type Conversion in JavaScript explained

A guide on how type casting works in JavaScript

Posted on April 05, 2021

In programming language, casting is a way of telling the compiler to change an expression or value from one type to another. At times, you may want to convert your JavaScript expressions or values from one type to another.

For example, you may want to convert a value of string type to number so that you can perform mathematical operations on the value. You may also want to convert a data of boolean type into a string type. JavaScript provides you with built-in global functions and methods to convert data from one type to another

In this tutorial, we’ll look at the following ways of casting type in JavaScript:

  • Casting data to string type
  • Casting data to number type
  • Casting data to boolean type

Let’s start with learning how to cast other types into string type first.

Casting data to string type

Number type to string type

To convert any type of data to string type, you need to use the built-in global function String(). For example, here’s how you convert a number type to a string type. Notice the log from the typeof operator call:

let str = String(101);

console.log(str); // "101"
console.log(typeof str); // "string"

Alternatively, you can also use the Number.toString() method to convert a number to string. It works just the same as the global String() function:

let str = (101).toString();

console.log(str); // "101"
console.log(typeof str); // "string"

Boolean type to string type

Next, you can also convert boolean type into string by using the global String() function or the Boolean.toString() method as follows:

let strOne = String(true);
console.log(strOne, typeof strOne); // "true" "string"

let strTwo = (true).toString();
console.log(strTwo, typeof strTwo); // "true" "string"

Date type to string type

You can even convert a value of Date object, which holds date values into string with the same global function String() and the Date.toString() method:

let strOne = String(new Date("2021-04-06"));
console.log(strOne, typeof strOne); 
// "Tue Apr 06 2021 00:00:00 GMT+0000 (Greenwich Mean Time)" "string"

let strTwo = (new Date("2021-03-05")).toString();
console.log(strTwo, typeof strTwo); 
// "Tue Apr 06 2021 00:00:00 GMT+0000 (Greenwich Mean Time)" "string"

Special falsy types to string type

There are also special falsy type that you can convert to string only by using the String() global function, such as null, undefined, and NaN:

String(null); // "null"
String(undefined); // "undefined"
String(NaN); // "NaN"

Let’s look at converting other data to number type next.

Casting data to number type

To cast a string into number type, you can use either the global function Number():

let num = Number("202");

console.log(num, typeof num); // 202 "number"

You can also convert other types like boolean with the Number() function. true value will be converted as 1 and false value will be converted as 0:

let numTrue = Number(true);
console.log(numTrue); // 1

let numFalse = Number(false);
console.log(numFalse); // 0

Finally, you can also convert a string that represents a floating number into number type using Number() function:

let num = Number("7.6");

console.log(num); // 7.6

And that’s all about casting other types to number type in JavaScript. Let’s take a look at casting to boolean type next.

Casting other types to boolean type

The global function Boolean() allows you to convert any other data type into boolean type. Here are some examples of converting a string and a number into boolean:

let boolStr = Boolean("string");
console.log(boolStr); // true

let boolNum = Boolean(9);
console.log(boolNum); // true

Any truthy values will be converted to true while falsy values will be converted into false.

See also: JavaScript truthy / falsy values explained


And now you’ve learned how JavaScript type casting works. The global function String(), Number(), and Boolean() enable you to convert your data from one type to another easily.

Also, since JavaScript is a weakly-typed language, so it also has a type coercion feature that automatically converts data from one type to another when the situation requires it.

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