object property accesses, response to tip#3 in https://gist.github.com/yuval-a/d5794cc1439c08af085f

object-property-access.js

// response to https://gist.github.com/yuval-a/d5794cc1439c08af085f
// via linkedin post https://www.linkedin.com/groups/121615/121615-6135714090246553602
// specifically tip #3,
// property === undefined is faster than hasOwnProperty(property)

// I'm not sure what the intent is, because the two checks compare different things...

var a = { name: 'aaaaaa' };
a.name === undefined //  false (expected)
a.hasOwnProperty('name') // true (expected)

// ...because inheritance via the prototype chain is the whole reason for `hasOwnProperty` 
// in the first place:

var b = Object.create(a);
b.name === undefined //  false (expected)
b.hasOwnProperty('name') // false (oh...)

// So, if you're only checking the _existence_ of the property, use the ('property' in object) 
// expression, _if_ existence is all you're checking:

'name' in a // true

// Otherwise if you're checking whether the _value_ of the property is undefined or null, 
// but not any other falsey value (0, false, NaN or the empty string ('')), then use the looser 
// equality operator, `==`.

// Here's a test object with some one-liner assertions below to show all this:

var test = {
  name: 'test name',
  n: null,
  u: undefined,
  x: !!0,
  y: '',
  z: 0
};

// Existence checks are not value checks. If a key exists its _value_ 
// can still be undefined
'bonk' in test // false (expected)
test.hasOwnProperty('bonk') // false (expected)

// These evaluate to true because the key 'u' exists on the test object
'u' in test // true
test.hasOwnProperty('u') // true

// check their values, or rather what their keys resolve to...
test.bonk == null // true
test.n == null // true
test.u == null // true

// whereas...
test.x == null // false
test.y == null // false
test.z == null // false