When to use function expression Vs. function declaration?

Hi! Function declarations and expressions differ in terms of hoisting and their effect on scope.

Function declarations are hoisted. This means that the functions are placed in memory before any code is executed. Thus, you can call the function before you declare it in your script; your function declarations have been virtually moved (i.e. 'hoisted') to the top of their scope. There can be various subtle benefits to this — one simple one is that it can help with the organisation of your code, in that you can place functions where they have the richest context in your script, not necessarily where they have to be in terms of availability.

Since functions expressions are not hoisted, they are run only when the interpreter reaches them. So, consider how this contrasts with your hoisted declarations. Function expressions compel more structured code, in a sense, because you need to be more mindul of order. Function expressions need to exist in code before the point at which they're called and, when you get into higher-order functions (passing functions to functions, returning functions from functions), this is something you really need to look out for. You'll soon encounter callbacks, so keep that in mind for then!

Function expressions can also be called immediately, resulting in so-called IIFEs (Immediately Invoked Function Expressions). IIFEs are useful for callbacks and scope management — there's no 'pollution' of your global scope because your variable declarations are isolated.

Hope this helps. You'll soon learn more about functions, I'm sure, and this will all make sense!