Pure functions using ES2015 to create common utilities

forEach.js

function forEach(arr, callback, start=0) {
  if (0 < start && start < arr.length) {
    callback(arr[start], start, arr);
    return forEach(arr, callback, start + 1);
  }
}

len.js

function len([first, ...rest]) {
  if (!first) {
    return 0
  } else {
    return 1 + len(rest);
  }
}

map.js

function map([first, ...rest], f) {
  if (!first) {
    return [];
  } else {
    return [f(first), ...map(rest, f)];
  }
}

range.js

function range (start, end) {
  return Array.from({length: (end - start)}, (v, k) => k + start);
}

replaceString.js

function replaceString(pattern, replacement, str) {
  return str.split(pattern).join(replacement);
}