What’s the double exclamation sign for in JavaScript?

If you have ever noticed a double exclamation mark (!!) in someone’s JavaScript code you may be curious what it’s for and what it does. It’s really simple: it’s short way to cast a variable to be a Boolean (true or false) value. Let me explain.

Use of double exclamation mark

Using a double exclamation point will instantly identify you as an over-excited internet poster under-familiar with the colloquial usage of English. (unless you’re writing in JavaScript, in which case it’s perfectly ok). The double exclamation point, or double bang, converts a truthy or falsy value to “true” or “false”. In other words, it operates exactly like Boolean(value) would.

The following values are considered by JavaScript to be falseys:

• Empty string: ""
• 0
• null
• undefined
• NaN

The following values are considered by JavaScript to be truthys:

• Object: {}
• Array: []
• Not empty string: "anything"
• Number other than zero: 3.14
• Date: new Date();

The JavaScript engine that is executing your code will attempt to convert (or coerce) a value to a boolean when necessary, such as when evaluated in an if-statement.

JavaScript is not a static language, rather it is a dynamic language. That means that a variable can reference or hold a value of any type, and further, that type can be changed at any point. Whether you prefer a static or dynamic language is for you to decide.

But, we can certainly have a notion of a type in JavaScript. Here is a quick list of the various data types in JavaScript:
- Boolean
- String
- Number
- Object

Coerce object to boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, true. A boolean data type is the simplest of all data types as it is a simple bit value: 0 (false) or 1 (true).

So !! is not an operator, it’s just the ! operator twice.

Let me explain with best example to detect IE version

let isIE8 = false; 
isIE8 = !! navigator.userAgent.match(/MSIE 8.0/); 
console.log(isIE8); // returns true or false

If you use below code

console.log(navigator.userAgent.match(/MSIE 8.0/));  // returns either an Array or null

Instead if you use !! result will be different

console.log(!!navigator.userAgent.match(/MSIE 8.0/));  // returns either true or false

Thank you for reading!