05 — Functions

One line: A function is an object like any other, stored, passed and given properties, and for every function except an arrow, this is chosen by the call rather than by the place the function was written.

The pages follow the questions a function raises, in the order you meet them. How do you write one? The three forms, and hoisting. What is this inside it? Whatever the call chose, unless it is an arrow or call, apply or bind chose for you. What does it take? Defaults, rest and arguments. What is it as a value? An object with properties, passed to and returned from other functions. What does a call cost? A stack frame, and Node stops a recursion that uses too many.

The pages

Page	Level	In one line	Status
Declarations, expressions and arrows	101	Only a function declaration is hoisted with its body, so declared() works on the line above function declared() {}; the same early call throws a TypeError for a var function expression and a ReferenceError for an arrow in a const.	stub
this	201	this is chosen by the call, not by where the function was written: user.greet() passes user, but const g = user.greet; g() passes undefined in a module and the global object in a sloppy CommonJS file.	stub
Arrow functions	201	An arrow function has no this of its own — it uses the this of the code around it, and call, apply and bind cannot change that — and it has no arguments and no prototype, so new on it throws a TypeError.	stub
call, apply and bind	201	call and apply choose this for one call; bind returns a new function whose this no later call or bind can change — yet new on a bound function still builds a fresh object and ignores the bound this.	stub
Parameters	101	A default parameter is evaluated again at every call that passes undefined — not null — so (item, list = []) gives each call a fresh array, where Python's lst=[] is built once and shared; a rest parameter is a real array, arguments is not.	stub
Functions are objects	101	A function is an object: it takes new properties, and its own length stops counting at the first default parameter — (a, b = 1, c) => {} has length 1 — while name is inferred from the variable an anonymous function is assigned to.	stub
Higher-order functions	101	A function that takes a callback decides what arguments the callback gets: ["1", "2", "3"].map(parseInt) is [1, NaN, NaN] because map also passes each index, which parseInt reads as a radix — and tsc accepts the call without a word.	stub
Recursion and the call stack	201	In Node 25 a one-line recursive function overflows after about 12,000 calls with a catchable RangeError; --stack-size moves that limit, but past the operating system's stack the process dies with a segfault, and the spec's proper tail calls never shipped in V8.	stub

Boundaries

Closures and scope are in Variables and scope; callbacks called later, async functions and promises are in Async and the event loop; types for parameters and return values are in Everyday types.