index.js

'use strict';

/**
 * Calculate the simple moving average of an array. A new array is returned with the average
 * of each range of elements. A range will only be calculated when it contains enough elements to fill the range.
 *
 * ```js
 * console.log(sma([1, 2, 3, 4, 5, 6, 7, 8, 9], 4));
 * //=> [ '2.50', '3.50', '4.50', '5.50', '6.50', '7.50' ]
 * //=>   │       │       │       │       │       └─(6+7+8+9)/4
 * //=>   │       │       │       │       └─(5+6+7+8)/4
 * //=>   │       │       │       └─(4+5+6+7)/4
 * //=>   │       │       └─(3+4+5+6)/4
 * //=>   │       └─(2+3+4+5)/4
 * //=>   └─(1+2+3+4)/4
 * ```
 * @param  {Array} `arr` Array of numbers to calculate.
 * @param  {Number} `range` Size of the window to use to when calculating the average for each range. Defaults to array length.
 * @param  {Function} `format` Custom format function called on each calculated average. Defaults to `n.toFixed(2)`.
 * @return {Array} Resulting array of averages.
 * @api public
 */

function sma(arr, range, format) {
  if (!Array.isArray(arr)) {
    throw TypeError('expected first argument to be an array');
  }

  var fn = typeof format === 'function' ? format : toFixed;
  var num = range || arr.length;
  var res = [];
  var len = arr.length + 1;
  var idx = num - 1;
  while (++idx < len) {
    res.push(fn(avg(arr, idx, num)));
  }
  return res;
}

/**
 * Create an average for the specified range.
 *
 * ```js
 * console.log(avg([1, 2, 3, 4, 5, 6, 7, 8, 9], 5, 4));
 * //=> 3.5
 * ```
 * @param  {Array} `arr` Array to pull the range from.
 * @param  {Number} `idx` Index of element being calculated
 * @param  {Number} `range` Size of range to calculate.
 * @return {Number} Average of range.
 */

function avg(arr, idx, range) {
  return sum(arr.slice(idx - range, idx)) / range;
}

/**
 * Calculate the sum of an array.
 * @param  {Array} `arr` Array
 * @return {Number} Sum
 */

function sum(arr) {
  var len = arr.length;
  var num = 0;
  while (len--) num += Number(arr[len]);
  return num;
}

/**
 * Default format method.
 * @param  {Number} `n` Number to format.
 * @return {String} Formatted number.
 */

function toFixed(n) {
  return n.toFixed(2);
}

/**
 * Expose `sma`
 */

module.exports = sma;