An explanation of why double negation is valuable.

notnot.js

var obj = {};

if (obj) {
  // this would run because obj is truthy. See https://developer.mozilla.org/en-US/docs/Glossary/Truthy and https://developer.mozilla.org/en-US/docs/Glossary/Falsy
}

if (!!obj) {
  // this would also run because obj is truthy.
  // when you negate it the first time, it becomes the value false
  // when you negate it the second time, it turn that false value into true
}

if (!!obj === true) {
  // this runs because the double negation results in a value of true
}

if (obj === true) {
  // this would NOT run, because obj (which evaluates to {}) does NOT equal the value true
}

// Here's an example of a function that expects a true value and demonstrates how the double negation is valuable
var resultFromSomeOtherFunction = {};

function aFunctionThatExpectsTrue(val) {
  if (val === true) {
    return 'nice!';
  } else {
    throw new Error('I don\'t understand the non-boolean');
  }
}

aFunctionThatExpectsTrue(resultFromSomeOtherFunction); // this would throw an error
aFunctionThatExpectsTrue(!!resultFromSomeOtherFunction); // this would return the string 'nice!'