Does javascript resolve variables within function definitions before execution?

Question

If one.js happens to finish loading before two.js even gets parsed, will x exist? Surely this is possible because once an asynchronous script has been loaded, it's immediately executed and blocks further parsing of the HTML file. Thereby blocking two.js before it defines x.

To my knowledge, JavaScript doesn't need to resolve variables within a function. Instead, it waits until the function is executed to determine it's existence. Since x.prt isn't used until the window and it's resources (including two.js) have loaded, it should be safe load both scripts asynchronously, right?

jsfiles.html:

...
<script src="lib/one.js" type="text/javascript" async="async"></script>
<script src="lib/two.js" type="text/javascript" async="async"></script>
...

one.js:

window.addEventListener("load", function() {
    x.prt("Will this print 100% of the time?");
}, false);

two.js

var x = {
    prt: function(str) {
        console.log(str + " Or will this definition not be seen?");
    }
};

Have you tried running the code in a web browser?

Freyja
– Freyja

2015-07-28 21:45:42 +00:00
Commented Jul 28, 2015 at 21:45 — Freyja
– Freyja, Commented Jul 28, 2015 at 21:45
It works. But I'm not sure if my reasoning is correct.

thelittlegumnut
– thelittlegumnut

2015-07-28 21:48:32 +00:00
Commented Jul 28, 2015 at 21:48 — thelittlegumnut
– thelittlegumnut, Commented Jul 28, 2015 at 21:48

user229044 · Accepted Answer · 2015-07-28 21:50:46Z

1

JavaScript doesn't attempt to evaluate anything inside a function until the function executes. It will not attempt to look up x until the function executes, at which point x will exist as a global variable.

If one.js happens to finish loading before two.js even gets parsed, will x exist?

No, but it doesn't matter, because you don't need x to exist to define a function which uses x. x will exist by the time your window's load callback fires and your callback tries to use x.

answered Jul 28, 2015 at 21:50

user229044♦

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1 Comment

thelittlegumnut Over a year ago

As a side question... Is there a benefit to using the var keyword in the global scope?

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