Rule 8.17

A local function (a const that holds an arrow function or a function expression, or a function declaration, written inside a function’s body) is a function of the module named after the body that declares it (surface §14), and it may read and write the variables of the functions around it, as a TypeScript closure does. A name its body reads is looked up as TypeScript looks it up, from the innermost block outward; one that lands on a let, a const or a parameter of a function around it is a capture, which the emitted function takes as a parameter ahead of its own and which every call of it passes: by value while neither it nor a local function it calls writes the variable, and by reference once one does; a const keeps its constant, so a loop it bounds is still counted (Rule 7.5). A whole write to a captured value parameter shares the local copy of Rule 8.8 with the enclosing body and every other closure; its reference never reaches the enclosing caller. this in an arrow function is the object of the method around it, captured the same way, and the class the call names in a static member (Rule 8.13). A write through a capture follows the variable’s own declaration: a let may be written, a const only through what it holds and only when its initializer built the value (Rule 6.10), and a whole-rebound value parameter through its mutable local copy (Rule 8.8). A function declaration may be called anywhere in its block, as TypeScript hoists it, and a local function may be declared in a generic function, once for each of its instances. A call of a local function at a point where a variable it captures is not declared yet must be refused, since TypeScript throws there; so must a function named where a value is read (held in a variable, returned, compared or chosen at run time), there being no function value, other than as an argument of a parameter that takes a function (Rule 8.18). A function named as the callback of a fold (any(xs, isBig), zip(xs, ys, f)) passes what it captures to every call the fold makes.