Introduction

In JavaScript, data types are used to classify one particular type of data, determining the values that you can assign to the type and the operations you can perform on it.

Although due to type coercion, JavaScript will automatically convert many values, it is often best practice to manually convert values between types in order to achieve expected results.

This tutorial will guide you through converting JavaScript’s primitive data types, including numbers, strings, and Booleans.

Implicit Conversion

As a programming language, JavaScript is very tolerant of unexpected values. Because of this, JavaScript will attempt to convert unexpected values rather than reject them outright. This implicit conversion is known as type coercion.

Some methods will automatically convert values in order to make use of them. The alert() method takes a string as its parameter, but it will automatically convert other types into strings. So, we can pass a number value to the method:

alert(8.5);

If we run the line above, the browser will return a pop-up alert dialog box that displays the 8.5 value except it will have been converted to a string in order to do so.

When using strings that can be evaluated to numbers with mathematical operators, you’ll find that JavaScript is able to handle the values by implicitly converting the strings to numbers, as shown in the examples below.

// Subtraction
"15" - "10";

Output
5

// Modulo
"15" % "10";

Output
5

However, not every operator will work as expected. The + operator is notably problematic as it can signify either addition or string concatenation.

// When working with strings, + stands for concatenation
"2" + "3";

Output
"23"

Since the + operator is multi-purpose, the string values of 2 and 3, despite being numerical strings, are concatenated to the string value of 23 rather than added together to be the number 5.

Because ambiguity can exist and sometimes cause unexpected results, it is often best to explicitly convert data types in your code as much as possible. This will help with managing input from users and handling errors.

Converting Values to Strings

Values can be explicitly converted to strings by calling either String() or n.toString().

With the String() function, let’s convert a Boolean value to a string by passing the value true into the parameters for String().

String(true);

When we do this, the string literal "true" will be returned.

Output
"true"

Alternatively, we can pass a number into the function.

String(49);

A string literal of that number will be returned.

Output
"49"

Let’s use the String() function with a variable. We’ll assign a number value to the variable odyssey and then use the typeof operator to check for type.

let odyssey = 2001;
console.log(typeof odyssey);

Output
number

At this point, the variable odyssey is assigned the numerical value of 2001, which we have confirmed to be a number.

Now, let’s reassign odyssey to its string equivalent and then use typeof to confirm that we have successfully converted the variable’s value from a number to a string.

odyssey = String(odyssey);  // "2001"
console.log(typeof odyssey);

Output
string

In the example above, we have confirmed that odyssey was reassigned to be equivalent to a string value following the data type conversion.

We can use n.toString() in a similar way. We can replace n with a variable:

let blows = 400;
blows.toString();

The variable blows will be returned as a string.

Output
"400"

Alternatively, we can put a value within parentheses rather than a variable with n.toString():

(1776).toString();          // returns "1776"
(false).toString();         // returns "false"
(100 + 200).toString();     // returns "300"

By using String() or n.toString() we are able to explicitly convert values of Boolean or number data types to string values in order to ensure that our code behaves as we anticipate.

Converting Values to Numbers

When converting values to a number data type, we’ll use the Number()method. Primarily, we’ll be converting strings of numerical text to numbers, but we can also convert Boolean values.

We can pass a string of a number to the Number() method:

Number("1984");

The string will be converted to a number and no longer be enclosed within quotation marks.

Output
1984

We can also assign a string to a variable and then convert it.

let dalmatians = "101";
Number(dalmatians);

Output
101

The string literal "101" was converted to the number 101 via its variable.

Strings of white spaces or empty strings will convert to 0.

Number(" ");    // returns 0
Number("");     // returns 0

Be aware that strings of non-numbers will convert to NaN which stands for Not a Number. This includes numbers separated by spaces.

Number("twelve");   // returns NaN
Number("20,000");   // returns NaN
Number("2 3");      // returns NaN
Number("11-11-11"); // returns NaN

For Boolean data types, false will evaluate to 0 and true will evaluate to 1.

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

The Number() method converts non-number data types to numbers.

Converting Values to Booleans

To convert numbers or strings to Boolean values, the Boolean() method is used. This can be useful for determining whether a user entered data into a text field or not, for example.

Any value that is interpreted as empty, like the number 0, an empty string, or values that are undefined or NaN or null are converted to false.

Boolean(0);         // returns false
Boolean("");        // returns false
Boolean(undefined); // returns false
Boolean(NaN);       // returns false
Boolean(null);      // returns false

Other values will be converted to true, including string literals composed of white space.

Boolean(2000);      // returns true
Boolean(" ");       // returns true
Boolean("Maniacs"); // returns true

Note that "0" as a string literal will convert to true since it is a non-empty string value:

Boolean("0");   // returns true

Converting numbers and strings to Boolean values can allow us to evaluate data within binary terms and can be leveraged for control flow in our programs.

Conclusion

This tutorial covered how JavaScript handles conversion of its primitive data types. Though due to type coercion, data types will implicitly convert in many cases, it is a good habit to explicitly convert data types in order to ensure that programs are functioning as expected.

To learn more about JavaScript’s data types, read “Understanding Data Types in JavaScript.” To see how data type conversion is done in other programming languages, take a look at “How To Convert Data Types in Python 3.”