Structuring behaviour.
WHAT A FUNCTION IS
A reusable block of code, taking inputs and returning a result.
WHAT SCOPE IS
Where a name is visible.
WHAT THE RULE IS
Names declared inside a block are visible only within it.
WHY THAT MATTERS
It prevents parts of the program interfering with each other.
WHAT A CLOSURE IS
A function retaining access to the scope it was created in, even after that scope has finished.
WHY THAT IS USEFUL
It allows private state, and functions that remember something.
WHY IT SURPRISES PEOPLE
Variables appear to persist beyond where they were defined.
WHAT ARROW FUNCTIONS DIFFER IN
Their shorter form, and how they treat the contextual reference to the current object.
WHY THAT MATTERS
The difference causes a specific and common class of bug.
WHAT MAKES A GOOD FUNCTION
It does one thing Its name says what It does not depend on hidden state
WHAT TO AVOID
Functions modifying things outside themselves Deeply nested functions Very long functions
