Consider this code:

Many developers would expect undefined, since the var a statement comes after the a = 2, and it would seem natural to assume that the variable is re-defined, and thus assigned the default . However, the output will be 2.

You might be tempted to assume that, since the previous snippet exhibited some less-than-top-down looking behavior, perhaps in this snippet, 2 will also be printed. Others may think that since the a variable is used before it is declared, this must result in a ReferenceError being thrown.

So, what’s going on here? It would appear we have a chicken-and-the-egg question. Which comes first, the declaration (“egg”), or the assignment (“chicken”)?