One reason why !! is dangerous

0-looks-like-correct.js

// _.find returns undefined if nothing is found, otherwise returns the found item.
var existsOnCurrentPeriod = _.find(currentValues.operators, function(operatorId) {
    return operatorId === operator.usbKeyId;
});

operator.existsOnPeriod = !!existsOnCurrentPeriod;

// This looks correct at first glance, but see below

1-think-of-this-scenario.js

// The same logic with slight variable name changes

var operatorIds = [0, 1, 2, 3];
var usbKeyId = 0;
var existsOnCurrentPeriod = _.find(operatorIds, function(id) {
    return id === usbKeyId;
});

var exists = !!existsOnCurrentPeriod;
console.log(exists);
> false

2-correct-solution.js

var operatorIds = [0, 1, 2, 3];
var usbKeyId = 0;
var existsOnCurrentPeriod = _.find(operatorIds, function(id) {
    return id === usbKeyId;
});

var exists = !_.isUndefined(existsOnCurrentPeriod);
console.log(exists);
> true