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JavaScript is partly an object-oriented language.

To learn JavaScript, we got to learn the object-oriented parts of JavaScript.

In this article, we’ll look at primitive wrappers.

Boolean

The Boolean can be used as a factory function or a constructor.

For instance, we can write:

const b = new Boolean();

and we get a boolean wrapper object returned.

We can convert it back to a primitive value with valueOf :

b.valueOf();

A better use of the Boolean function is to use it as a factory function.

For example, we can write:

Boolean("foo");

then we get true .

It’ll return true for anything truthy and false otherwise.

Boolean without new returns a primitive value.

Number

Number can also be used as a constructor or a factory function.

For instance, if we have:

const n = Number('12');

then we return the number 12.

And if we have:

const n = new Number('12');

then we return an object with value 12.

So we should use it without new to eliminate confusion.

Number also has some static properties.

Number.MAX_VALUE is 1.7976931348623157e+308 .

There’s also the Number.POSITIVE_INFINITY which is Infinity .

And Number.NEGATIVE_INFINITY which is -Infinity .

The toFixed method formats a number to a decimal number with the given number of fractional digits.

For instance, we can write:

(123.456).toFixed(1);

We have “123.5” .

The toExponential method returns the number string in exponential notation.

For instance, if we have (12345).toExponential() , then we get:

"1.2345e+4"

The toString method lets us convert numbers to number strings of various bases.

For instance, we can write:

(255).toString(2)

Then we convert the number to the binary string:

"11111111"

And we can write:

(255).toString(16)

and get:

"ff"

To convert it to a decimal, we write:

(255).toString(10)

And we get:

"255"

String

The String function is used to create a string.

We can use it as a constructor or factory function.

We don’t want to create an object with it to reduce confusion, so we should use it as a factory function.

For instance, we can write:

const obj = new String('foo');

then we create a string wrapper object.

To convert it to a primitive value, we call valueOf :

const s = obj.valueOf();

It’s better just to create a string as a string literal:

const s = 'foo';

We can also use the function to convert non-strings to strings:

const s = String(123);

We can get the number of characters in a string with the length property.

So if we have:

const s = 'foo';

Then s.length is 3.

If we pass in an object to String , then the toString method will be called.

So:

String({
  a: 1
});

returns:

"[object Object]"

And:

String([1, 2, 3]);

returns:

"1,2,3"

Conclusion

Boolean, Number, and String functions let us create primitive values.

We should use them as factory functions to convert between different primitive types.