Simple impl of lodash.chain

chain.js

function map(arr, fn) {
  console.log('invoked map');
  if (arr) {
    return arr.map(fn);
  }
  return [];
}

function filter(arr, fn) {
  console.log('invoked filter');
  if (arr) {
    return arr.filter(fn);
  }
  return [];
}

function reduce(arr, fn, def) {
  console.log('invoked reduce');
  if (arr) {
    return arr.reduce(fn, def);
  }
  return def;
}

function chain(arr) {
  const ctx = {};
  const stack = [() => arr];
  const pushOrigin = stack.push.bind(stack);
  stack.push = (elm) => {
    console.log('push', stack);
    pushOrigin(elm);
    return ctx;
  };
  
  ctx.value = () => stack.pop()();
  
  ctx.map = (fn) => stack.push(
    () => map(ctx.value(), fn)
  );
  
  ctx.filter = (fn) => stack.push(
    () => filter(ctx.value(), fn)
  );
    
  ctx.reduce = (fn, def) => stack.push(
    () => reduce(ctx.value(), fn, def)
  );
  
  return ctx;
}

console.log(
  chain([1, 2, 3])
    .map(n => n * 4)
    .filter(n => n > 5)
    .reduce((mem, item) => item + mem, 0)
    .value()
)