I was writing some vanilla JavaScript this weekend - it has been quite awhile since I did so. As I've been writing mostly C# code lately, what always gets me with JavaScript isn't its pain points or subtle nuances. It's the different behavior from compiled languages I'm often used to, like C# or Java. No matter how often I work in JS, as long as I'm committed to being a polyglot, it'll trip me up to the point that it's just a little bit of an annoyance.

Take this seemingly simple JavaScript code. What will be output to the browser console?

var myString = 'I am outside the function';
function myFunction() {
    console.log(myString);
    var myString = 'I am inside the function';
};
myFunction();

Was your guess I'm outside the function? Or was it I'm inside the function? I hate to let you down, but in either case you would be incorrect.

In your favorite browser's developer tools, the console will log undefined. But why?

In JavaScript, functions create brand new scopes. If we condense the example a little:

function myFunction() {
    // new scope, the variable is inaccessible outside the function
    var myString = 'I am inside the function';
}
myFunction();
console.log(myString); // undefined!

Of course, if you've worked in JavaScript (or basically any programming language) you can't expect to declare a variable without an assignment and expect to get anything back but null or, in JavaScript's case, undefined when I do something like this:

var dave;
console.log(dave); // will log undefined

So, if we look at this again:

var myString = 'I am outside the function';
function myFunction() {
    console.log(myString);
    var myString = 'I am inside the function';
};
myFunction();

To take all we've learned, we need to be aware of how something called hoisting works. For our purposes just know this: variable declarations in JS are always hoisted to the top of the current scope -- variable assignments are not.

So here, the declaration is hoisted to the top of the scope - or, in our case, the myFunction() function. So before the console.log statement in the function, you can basically envision a var myString in the first line of the function. And, of course, knowing this now it is clear to see why the logging statement will come back as undefined.

Just remember this: always define your variables at the top of the current scope. Always.