How is the Google Analytics Tracking code working? (async method call?)

问题

This code is the Google Analytics tracking code. How is not make an error because of undifened ga method?

(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
})(window,document,'script','//www.google-analytics.com/analytics.js','ga');

ga('create', 'UA-XXXX-Y', 'auto');

ga('send', 'pageview');

回答1:

Expanded, the script looks like this:

(function(i, s, o, g, r, a, m) {
    i['GoogleAnalyticsObject'] = r;
    i[r] = i[r] || function() {
        (i[r].q = i[r].q || []).push(arguments)
    }, i[r].l = 1 * new Date();
    a = s.createElement(o),
    m = s.getElementsByTagName(o)[0];
    a.async = 1;
    a.src = g;
    m.parentNode.insertBefore(a, m)
})(window, document, 'script', '//www.google-analytics.com/analytics.js', 'ga');

ga('create', 'UA-XXXX-Y', 'auto');

ga('send', 'pageview');

You'll notice that 'ga' is the fifth parameter passed to the function, and window is the first. Inside the initialization, the first couple lines set i[r], aka window['ga'] to a function. And there you go!

来源：https://stackoverflow.com/questions/23474151/how-is-the-google-analytics-tracking-code-working-async-method-call