Checking existence of properties in JavaScript

问题:

I'm new to JavaScript and a little bit confused with the duck typing concept. As far as I can tell, I understood the concept. But that leads to a strange consequence in my thoughts. I will explain with the following example:

I'm currently working on a mobile web app with jQuery Mobile. At one point I capture the vmousedown event for a canvas. I'm interested in the pressure of the touch. I found the Touch.webkitForce property.

$('#canvas').live('vmousedown', function(e){
    console.log(e.originalEvent.originalEvent.touches[0].webkitForce);
}

This works fine when using the Remote Debugging for Chrome. But throws an exception when testing in Opera Firefly, because the originalEvent property is no touch event, but a click event.

So every time I access a property of an object which is not under my authority, do I have to check existence and type?

if( e.originalEvent &&
    e.originalEvent.originalEvent &&
    e.originalEvent.originalEvent.touches && 
    e.originalEvent.originalEvent.touches[0] && 
    e.originalEvent.originalEvent.touches[0].webkitForce) {

    console.log(e.originalEvent.originalEvent.touches[0].webkitForce);
}

Can please someone clarify that for me?

回答1:

So every time I access a property of an object which is not under my authority, do I have to check existence and type?

Yes you will have to check the whole path, once at a time, or you can automate it:

function deepObject(o, s) {
    var ss = s.split(".");

    while( o && ss.length ) {
        o = o[ss.shift()];
    }

    return o;
}

var isOk = deepObject(e, "originalEvent.originalEvent.touches.0.webkitForce");

if ( isOk ) {
    // isOk is e.originalEvent.originalEvent.touches.0.webkitForce;
}

---

Test case:

var o = {
  a: {
    b: {
      c: {
        d: {
          e: {
          }
        }
      }
    }
  }
}

var a = deepObject(o, "a.b.c");
var b = deepObject(a, "d");

console.log(a); // {"d": {"e": {}}}
console.log(b); // {"e": {}}
console.log(deepObject(o, "1.2.3.3")); // undefined

回答2:

Use try catch

$('#canvas').live('vmousedown', function(e) {
   try {
       console.log(e.originalEvent.originalEvent.touches[0].webkitForce);
   } catch(e) {
       console.error('error ...');
   }
}

回答3:

As you are using a specific framework to capture your events, i think that you should assume that the originalEvent is always defined. If it isn't, then it is probably a good thing to throw an error as something clearly went wrong somewhere in the capture of the event.

However, the event could be a MouseEvent or a TouchEvent, also, the webkitForce property may not be supported. These are the kind of cases that you might want to detect :

// assume that originalEvent is always be defined by jQuery
var originalEvent = e.originalEvent.originalEvent;
if (originalEvent instanceof TouchEvent) {  // if touch events are supported
  // the 'touches' property should always be present in a TouchEvent
  var touch = originalEvent.touches[0];
  if (touch) {
      if (touch.webkitForce) {
        // ...
      } else {
        // webkitForce not supported
      }
  }  // else no finger touching the screen for this event
} else {
   // probably a MouseEvent
}