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var factorial = function(num) {
  return !num ? 1 : num *= factorial(num - 1);
}

// Shorter ES6 variant
// const factorial = num => !num ? 1 : num *= factorial(num-1);

alert(factorial(6)); // 720
// https://jsfiddle.net/2ary59t0/157/

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const newArray = (size) => [...Array(size).keys()];
newArray(5); // [0, 1, 2, 3, 4]

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const memoize = function(fn) {
  const cache = {};
  return function(...args) {
    const key = args.join();
    if (cache[key]) {
      console.log('from cache!');
      return cache[key];
    }
    return cache[key] = fn.apply(this, args);
  };

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ES6 JavaScript Compose Function

Definition

const compose = (...fns) =>
  fns.reduce((prevFn, nextFn) =>
    (...args) => prevFn(nextFn(...args)),
    (value) => value);

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const highlightText = (str, match) => {
  if (!str) return null;
  if (!match) return str;
  const regex = new RegExp(match, 'mig');
  return `>${str}<`.replace(/>[^<]+</gmi, (match) => match.replace(regex, '[$]')).slice(1, -1);
};

console.log(highlightText('lorem 0 erty o', 'o'));
console.log(highlightText('<p class="lorem">lorem ipsum dolor sit amet</p>', 'o'));
// l[$]rem 0 erty [$]