How does the double exclamation mark (‼) work in JavaScript?

General Discussions
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1

I’m looking at some code in the Discover Meteor demo and am trying to understand the use of return !!userId;. Here’s the section of code:

Posts.allow({
  insert: function(userId, doc) {
    // Only allow posting if you are logged in
    return !!userId;
  }
});

Can someone explain how the double exclamation mark in JavaScript works in this context and what effect it has on the userId value?

2

Well, it’s actually a pretty neat little trick in JavaScript to convert any value into a Boolean. When you use !!value, it coerces the value to either true or false. The first exclamation mark negates the value, and the second one turns it back into the corresponding Boolean. It’s a quick way to check if something is truthy or falsy in your code.

3

Why not just use if (userId)? Sure, you could! But using !!userId is especially handy when you’re returning a value rather than doing a direct conditional check. For example, try this:

return !!userId;

It always returns exactly true or false, making it super clear that you’re expecting a Boolean result. This is important in cases where you want to be explicit about your return type, ensuring that you’re not accidentally returning a string, object, or something else that might behave weirdly as a truthy value. It’s all about the clarity and precision that the double exclamation mark in JavaScript brings.

4

Absolutely! It’s also great for cleaning up checks. Let’s say you’re checking a form input:

let isFilled = !!input.value;

In this case, isFilled will be true if the input is not empty and false if it is. It’s a super compact way to get a reliable Boolean without writing longer, more complicated if-statements. This is where the double exclamation mark in JavaScript feels like a little bit of magic—it simplifies your code, keeps things neat, and ensures you always get a proper Boolean without any hassle.