Interpretation of javascript code - Tilde symbol in front of ternary IF operator

Question

Interpretation of javascript code - Tilde symbol in front of ternary IF operator

I was checking the code of respons.js in express and came across this code:

res.contentType =
res.type = function(type){
  return this.set('Content-Type', ~type.indexOf('/')
    ? type
    : mime.lookup(type));
};

My question is what does the ~ operator do in front of the type.indexOf() statement? What is its purpose and when is it used?

javascript
node.js
express

Answer 1

It is a bitwise NOT, although its use here is quite opaque.

It is being used to transform a -1 result from indexOf (i.e., string not found) into a 0, which is a falsy value (since ~-1 == 0, and 0 is false in a boolean context), and it lets all other values remain truthy.

It could have been written more clearly as (type.indexOf('/') != -1) ? ... : ...

In plain English, it says, "Treat a -1 result (i.e., if / was not found) from indexOf as false; otherwise, treat the result as true".

Answer 2

The tilde is the bitwise NOT operator, just as ! is the logical NOT operator. You may want to take a look at the documentation of the operator on Mozilla Developer Network for its full usage and meaning.