Polyfills, this/call/apply/bind, Currying, Debounce, Promises & Coding Challenges

Loop, apply callback, return the right shape.

myMap / myFilter: for loop over this, call callback.call(thisArg, this[i], i, this), push results. myFilter guards with the boolean.
myReduce: default acc to initialValue; if undefined, seed acc = this[0] and start at index 1. Throw on empty array with no initial.
myForEach: loop, call callback, return nothing — it’s undefined on purpose.
myFlat: recursive helper flatten(arr, depth) — recurse if item is array and depth > 0.

bind returns a function; call/apply invoke immediately.

myBind: capture this as fn, return a new function that does fn.apply(context, [...args, ...newArgs]). Partial application for free.
myCall: attach this to context via a Symbol() key, invoke context[key](...args), delete the key, return the result.
myApply: same as myCall but accepts an array of arguments instead of variadic.
Default context to globalThis; using a Symbol key avoids overwriting existing properties.

Three tools, one job — control this.

call(ctx, a, b) invokes immediately with individual args.
apply(ctx, [a, b]) invokes immediately with an array of args.
bind(ctx, a) returns a new function with this locked — supports partial application.
Standalone calls set this to the global object (or undefined in strict mode); method calls set this to the owner.

Turn f(a,b,c) into f(a)(b)(c).

Generic curry: keep collecting args until args.length >= fn.length, then fn.apply(this, args).
Infinite currying (sum(1)(2)(3)...()): return a function that, if called with no arg, returns the accumulator; otherwise recurse with sum(a + b).
Powered entirely by closures — each returned function closes over the accumulated args.
Use fn.length (number of declared parameters) to know when enough args are collected.

Four closures that show up in every interview.

Debounce: clearTimeout then setTimeout(fn, delay) in closure — last call wins after silence.
Throttle: boolean flag inThrottle + setTimeout to clear it — first call per interval wins.
Once: boolean called + cached result — returns cached value on every subsequent call.
Memoize: Map cache keyed by JSON.stringify(args) — returns cached result for identical arguments.

The event loop rule on every interview whiteboard.

Step 1: all sync code — including Promise constructor bodies. resolve() does NOT exit the constructor.
Step 2: drain the entire microtask queue (promise .then, await resume, queueMicrotask).
Step 3: take one macrotask (setTimeout, setInterval, I/O) and run it.
Step 4: drain microtasks again. Repeat 3–4 forever.

Array method polyfills