++someVariable vs. someVariable++ in JavaScript

Same as in other languages:

• ++x (pre-increment) means “increment the variable; the value of the expression is the final value”
• x++ (post-increment) means “remember the original value, then increment the variable; the value of the expression is the original value”

Now when used as a standalone statement, they mean the same thing:

x++;
++x;

The difference comes when you use the value of the expression elsewhere. For example:

x = 0;
y = array[x++]; // This will get array[0]

x = 0;
y = array[++x]; // This will get array[1]