assuming x is an object... Is there any benefit of doing:
if (typeof x.foo != "undefined")
vs. doing
if (x.foo)
?
This question came up as I was reading this blog post: http://www.nczonline.net/blog/2010/03/09/custom-events-in-javascript/
In his example, he does:
function EventTarget(){
this._listeners = {};
}
EventTarget.prototype = {
constructor: EventTarget,
addListener: function(type, listener){
if (typeof this._listeners[type] == "undefined"){
this._listeners[type] = [];
}
this._listeners[type].push(listener);
In this case this._listeners[type] will never be anything except an array-- so is it not true tha开发者_如何学Got it would be cleaner in this case to just do
addListener: function(type, listener){
if (!this._listeners[type]){
this._listeners[type] = [];
}
this._listeners[type].push(listener);
?
Also, as a side question, I don't get why he's doing:
EventTarget.prototype = {
constructor: EventTarget
Isn't the constructor by default already set to EventTarget ('this') when you call new EventTarget() ?
Watch out for truthy values.
if (x.foo)
will not run if x.foo is
- false
- null
- undefined
- ""
- 0
- NaN
Where as if (typeof x.foo !== "undefined") {
only checks for whether the value is undefined
Alternative checks would be
if (x.foo !== undefined) {
and if (x.foo !== void 0) {
Do be wary that undefined
can be overwritten as a local variable
undefined = true
is a valid statement and will break all your code. Of course you will never see this code in production so you don't really have to shield against it, it's just something to be wary of.
I personally tend to use
if (x.foo != null) {
...
}
a lot which checks for both null
and undefined
.
[[Edit]]
In your specific example it's either an Array
or undefined
so !foo
is safe. personally I prefer to check specifically for undefined
so that users know I only expect it to run when it's undefined rather then when it's null
or false
or ""
. This makes the code more explicit / self documenting.
As to
EventTarget.prototype = {
constructor: EventTarget
If you overwrite EventTarget.prototype
with a new object then the EventTarget.prototype.constructor
property is lost and needs to be set again.
You do not need to set .constructor
again if you just extend the prototype by calling EventTarget.prototype.method = ...
.
The first explicitly checks that x.foo
is undefined
, whereas if (x.foo)
is checking to see if x.foo
is truthy.
http://11heavens.com/falsy-and-truthy-in-javascript
精彩评论