JavaScript Funky Behaviours

Gotchas.js

(function () {
  'use strict';
  
  var u;                      //undefined
  //Declares a variable, but does not define a value for it.
  
  /*
  ** Falsy values:
  **  • false     Surprise!
  **  • 0         Digit zero
  **  • ""        Empty string
  **  • undefined
  **  • null
  **  • NaN       A special Number value meaning Not-a-Number!
  **
  ** Truthy values: All other values are truthy, including "0" (zero in quotes),
  ** "false" (false in quotes), empty functions, empty arrays, and empty objects.
  */
    
  // all pass this test 
  console.log(!false==true);  // true
  console.log(!0==true);      // true
  console.log(!""==true);     // true
  console.log(!u==true);      // true
  console.log(!null==true);   // true
  console.log(!NaN==true);    // true
  
  // but falsy is not always false
  console.log("-");
  console.log(false==false);  // true
  console.log(0==false);      // true
  console.log(0===false)      // false
  console.log(""==false);     // true
  console.log(u==true);       // false
  console.log(u==false);      // false WAT?!?
  console.log(null==false);   // false WAT?!?
  console.log(NaN==false);    // false WAT?!?
  
  //take home
  // 1) Use negation with falsy values. Don't use == or !=
  //   if (!falsy) // good 
  //   if (falsy==false || falsy!=true) // won't work as expected
  
  // Weird cases
  console.log("-");
  console.log(undefined==null)  // true
  console.log(undefined===null) // false
  console.log("0"==0)           // true
  console.log("0"===0)          // false
  console.log([]==true);        // false
  console.log([]==false);       // true
  console.log([]===true);       // false
  console.log([]===false);      // false
  
  // Apparently normal…
  console.log("-");  
  if([]) console.log("truthy"); // truthy 
  if(![]) console.log("falsy"); // not falsy
  
  var isItTruthy = "truthy?";
  
  if([]) {
    isItTruthy = "yes";         // this runs
  }
  console.log(isItTruthy);      // yes
  // …or not?
  if ([]==true) {
    isItTruthy = "no";          // this does not run
  }
  console.log(isItTruthy);      // truthy

  if ([]==false) {
    isItTruthy = "yesss";       // this does run
  }
  console.log(isItTruthy);      // yesss
})()