Introduction
JavaScript is a dynamically typed language where bindings (or variables)
can hold any type at any point in time or can be changed from one type
to another. Within the language, there are different types such as
Number, String, Object, and Boolean.
Therefore, if a binding holds a Number type, it can be changed or
coerced into another data type like String or Boolean. To make it
possible, there are built-in methods that allow us convert binding types
such as Number(), Boolean(), String(). Also, JavaScript has a
language feature called type coercion.) which automatically
converts values from one data type to another.
However, with Boolean, we can cast a binding (or variable) to be a
Boolean type explicitly using the !! operator and change it from one
data type to a Boolean type. In this article, we will discuss how to use
the double exclamation mark (!!) or operator, and how it works.
Using double exclamation mark in JS
Within the JavaScript environment, other data type values can be cast to
be a Boolean type, and you can see this when we pass values as
conditional statements in a if/else statement.
Let’s illustrate by creating some if/else statement.
if ("") {
console.log("True");
} else {
console.log("False");
}
if ("121") {
console.log("True");
} else {
console.log("False");
}
if (1211) {
console.log("True");
} else {
console.log("False");
}
if (0) {
console.log("True");
} else {
console.log("False");
}
if ({}) {
console.log("True");
} else {
console.log("False");
}
False
True
True
False
True
From the output values, you can see that an empty string (””), zero
(0) are coerced to false values and non-empty strings, objects, and
numbers other than zero are coerced to true values.
Other than these values, null, undefined and NaN values are
coerced to false values. Also, array and date values are coerced
to false values.
Knowing this, we might want to explicitly coerce a binding value to be
of the Boolean type. To achieve this, we can make use of the double
exclamation mark - !!.
const str = !!"345";
console.log(str, typeof str);
const na = !!NaN;
console.log(na, typeof na);
console.log(!!"");
Output
true boolean
false boolean
false
So, the !! operator converts the "345" and NaN values to Boolean
data types based on the logic of type (Boolean) coercion
Also, you can achieve this using the Boolean method
const str = Boolean("345");
console.log(str, typeof str);
Output
true boolean
Summary
To coerce values of a binding to Boolean type, we can make use of the
double exclamation mark and the Boolean method. This allows us to
explicitly coerce the data to the Boolean type without relying on the
automatic conversion offered by the JavaScript environment.
References
Boolean - JavaScript | MDN
(mozilla.org)
Type coercion - MDN Web Docs Glossary:
Definitions of Web-related terms | MDN (mozilla.org).)
