JavaScript object detection: dot syntax versus ‘in’ keyword

• if(object.property)

  will fail in cases it is not set (which is what you want), and in cases it has been set to some falsey value, e.g. undefined, null, 0 etc (which is not what you want).

  var object = {property: 0};
  if(object.isNotSet) { ... } // will not run
  if(object.property) { ... } // will not run
  

• if('property' in object)

  is slightly better, since it will actually return whether the object really has the property, not just by looking at its value.

  var object = {property: 0};
  if('property' in object) { ... } // will run
  if('toString' in object) { ... } // will also run; from prototype
  

• if(object.hasOwnProperty('property'))

  is even better, since it will allow you to distinguish between instance properties and prototype properties.

  var object = {property: 0};
  if(object.hasOwnProperty('property')) { ... } // will run
  if(object.hasOwnProperty('toString')) { ... } // will not run
  

I would say performance is not that big of an issue here, unless you’re checking thousands of time a second but in that case you should consider another code structure. All of these functions/syntaxes are supported by recent browsers, hasOwnProperty has been around for a long time, too.

Edit: You can also make a general function to check for existence of a property by passing anything (even things that are not objects) as an object like this:

function has(obj, prop) {
    return Object.prototype.hasOwnProperty.call(obj, prop);
}

Now this works:

has(window, 'setTimeout'); // true

even if window.hasOwnProperty === undefined (which is the case in IE version 8 or lower).