JavaScript Primitive Wrapper Types

Summary: in this tutorial, you will learn about the primitive wrapper types including Boolean, String, and Number.

Introduction to primitive wrapper types #

JavaScript provides three primitive wrapper types: Boolean, Number, and String types.

The primitive wrapper types make it easier to use primitive values including booleans, numbers, and strings.

See the following example:

let language = 'JavaScript';
let s = language.substring(4);
console.log(s);  // ScriptCopy

In this example, The variable language holds a primitive string value. It doesn’t have any method like substring(). However, the above code works perfectly.

When you call a method on a variable that holds a number, a string, or a boolean, JavaScript performs the following steps behind the scenes:

• Create an object of a corresponding type.
• Call a specific method on the instance.
• Delete the instance immediately.

So the following code

let language = 'JavaScript';
let str = language.substring(4);Copy

is technically equivalent to the following code:

let language = 'JavaScript';
// behind the scenes of the language.substring(4);
let tmp = new String(language);
str = temp.substring(4);
temp = null;Copy

Primitive wrapper types vs. reference types #

When you create an object of a reference type using the new operator, the object will stay in the memory until it goes out of scope.

The following variable s will stay on the heap until it goes out of the scope:

let s = new String('JavaScript');
console.log(s);Copy

However, an automatically created primitive wrapper object exists for one line of code only. See the following example:

let s = 'JavaScript';
s.language = 'ECMAScript';
console.log(s.language); // undefinedCopy

In this example, we attempted to access the language property of the s variable and received a value of undefined instead of 'ECMAScript':

console.log(s.language); // undefinedCopy

The reason is that the following code creates a String object and assigns a value to the language property.

s.language = 'ECMAScript';Copy

However, the String object with the language property only exists during the execution of this line of code.

It’s not recommended to explicitly create primitive wrapper objects like the following:

let n = new Number(10);
let s = new String('JS');
let b = new Boolean(false);Copy

However, you should know which methods are available for a primitive value in order to manipulate it more effectively.

In this tutorial, you have learned about the JavaScript primitive wrapper types for Booleans, numbers, and strings.