{"id":2560,"date":"2016-02-11T13:25:41","date_gmt":"2016-02-11T13:25:41","guid":{"rendered":"http:\/\/www.nikola-breznjak.com\/blog\/?p=2560"},"modified":"2016-02-11T13:29:47","modified_gmt":"2016-02-11T13:29:47","slug":"why-doesnt-window-something-throw-an-error","status":"publish","type":"post","link":"https:\/\/nikola-breznjak.com\/blog\/stack-overflow\/why-doesnt-window-something-throw-an-error\/","title":{"rendered":"Why doesn’t window.something throw an error?"},"content":{"rendered":"

In this StackOverflow question I asked why doesn’t window.something throw an error<\/p><\/blockquote>\n

\"profile<\/a>
\nI’m a big fan of Stack Overflow<\/a> and I tend to contribute regularly (am currently in the top 0.X%<\/a>).\u00a0In this category (stackoverflow<\/a>)\u00a0of posts, I will be posting my top rated questions and answers. This, btw, is allowed as explained in the meta thread here<\/a>.<\/p>\n

As you may know, I’m really into Ionic framework lately\u00a0and am helping out on StackOverflow with the knowledge I gained so far with the framework. I’m currently #3<\/strong> in the top All time answerers<\/a> list.<\/p>\n

I actually asked this question<\/a>\u00a0myself:<\/p>\n

I’m using an AdMob plugin in my Ionic application, and one way, as you can see in the documentation<\/a>, to test if the plugin is properly loaded is to do:<\/p>\n

if(AdMob) {\r\n    \/\/\/other config code...\r\n}<\/pre>\n
Now, this works perfectly fine on a device<\/strong>. However, it doesn’t work in the browser<\/strong>; it throws an error in the console log: AdMob is not defined<\/code>.<\/p>\n
I have found a solution to test the existence of plugin like this (without throwing an error in the console):<\/p>\n
if (window.AdMob){...}<\/pre>\n
And I have seen this usage on multiple questions here on StackOverflow. However, I wasn’t able to find an explanation to as why<\/strong> this doesn’t throw an error.<\/p>\n
I have a vague reasoning to as why this would be so, but I would really appreciate it if someone experienced could explain it in more detail.<\/p>\n
edit<\/strong>: I made additional tests like this:<\/p>\n
var a = \"hi\";\r\nconsole.log(a); \/\/shows \"hi\"\r\nconsole.log(b); \/\/throws an error that b is not defined\r\n\r\nvar c = {};\r\nc.b = \"hi again\";\r\nconsole.log(c.b); \/\/shows \"hi again\" as expected\r\n\r\n\/\/and now for the grand finale\r\nconsole.log(c.something);\/\/doesn't throw an error, please explain to me in more detail why?<\/pre>\n
The answer<\/a>\u00a0by T.J. Crowder<\/a>\u00a0was:<\/p>\n
\n
I wasn’t able to find an explanation to as why this doesn’t throw an error.<\/p><\/blockquote>\n
In the first example, you’re trying to read the value of a completely undefined identifier. In the second example, you’re trying to read a property from an object that the object may not have.<\/p>\n
Trying to read the value of an undefined identifier is a ReferenceError<\/code>; the JavaScript engine has no idea what that identifer is. In contrast, trying to read the value of a property that the object doesn’t have yields the value undefined<\/code>.<\/p>\n
It’s just how the language is designed, where Brendan Eich drew the line: It’s okay to read the value of a non-existant property from an object, but not okay to read the value of an undeclared identifier.<\/p>\n
I should point out a third option: typeof<\/code>. You’re allowed to provide an undefined identifier as the operand to typeof<\/code>:<\/p>\n
if<\/span> (<\/span>typeof<\/span> AdMob<\/span> ===<\/span> \"undefined\"<\/span>)<\/span><\/code><\/pre>\n
That won’t throw a ReferenceError<\/code> even if AdMob<\/code> is undeclared; instead, typeof<\/code> will yield \"undefined\"<\/code>. (It will also yield \"undefined'<\/code> if AdMob<\/code> is a declared identifier with the value undefined<\/code> in it.)<\/p>\n
In a comment on another answer, you said:<\/p>\n
…it would just indeed help to see the exact official specification which confirms this.<\/p><\/blockquote>\n
That would be the ECMAScript specification<\/a>, specifically \u00a76.2.3.1<\/a> for throwing a ReferenceError<\/code> on an unresolvable symbol, and \u00a79.1.8<\/a> for returning undefined<\/code> for a property that doesn’t exist. But I should warnin you that the spec, especially this 6th edition spec, is very<\/strong> heavy going. \ud83d\ude42<\/p><\/blockquote>\n
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Why doesn\u2019t #window<\/a>.something throw an #error<\/a> in @ionicframework<\/a>? #javascript<\/a> https:\/\/t.co\/e0vMYiHJAU<\/a> pic.twitter.com\/lPHcStpDA5<\/a><\/p>\n
— Nikola Bre\u017enjak (@HitmanHR) February 11, 2016<\/a><\/p><\/blockquote>\n