Number.isNaN() polyfill

Number-isNaN-polyfill.js

// Number.isNaN() polyfill
!(typeof Number.isNaN == 'function') ||
  (Number.isNaN = function (value) {
    return value !== null // Number(null) => 0
      && (value != value // NaN != NaN
        || +value != value // Number(falsy) => 0 && falsy == 0...
      )
  });

// see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN#Description
// for explanation of coercions (false, '0', '', null, and undefined)

console.warn(
  [2, 3, 5, 7, 
   -0, 
   Number.NEGATIVE_INFINITY, 
   Number.POSITIVE_INFINITY, 
   +false, // +false => 0
   +true,
   false, // Number(false) => 0
   true,
   '0', // Number('0') => 0
   '1',
   '', // Number('') => 0
   '       ',
   null // Number(null) => 0
  ]
  .map(function(value) {
    return !Number.isNaN(value);
  })
  .filter(function(ok, i) {
    console.log(ok)
    return ok || (function(i) {
      throw 'test ' + (i) + ' failed.'
    }(i))
  })
);

console.warn(
  [undefined, // +undefined => NaN
   'jimmy',   // +'non-digit-string' => NaN
   1 * 'a',   // multiplication on non-numeric value, 
   NaN        // because NaN !== NaN, of course
  ]
  .map(function(value) {
    return Number.isNaN(value);
  })
  .filter(function(ok, i) {
    return ok || (function(i) {
      throw 'test ' + (i) + ' failed.'
    }(i))
  })
);