dracon · January 17, 2020 07:04 · Oct 14, 2019
diff --git a/fp.js b/fp.js
@@ -0,0 +1,90 @@
+/**
+ * Performs right-to-left composition, combining multiple functions into a single function.
+ * @function compose
+ * @param {...Function} args
+ * @returns {Function}
+ */
+const compose = (...fns) => x => fns.reduceRight((output, fn) => fn(output), x, fns);
+
+/**
+ * Performs left-to-right composition, combining multiple functions into a single function. Sometimes called `sequence`. Right-to-left composition is typically known as `compose`.
+ * @function pipe
+ * @param {...Function} args
+ * @returns {Function}
+ */
+const pipe = (...fns) => x => fns.reduce((output, fn) => fn(output), x);
+
+/** Runs the given function with the supplied object, then returns the object.
+ * @function tap
+ * @param {Function} f
+ * @returns {*}
+ */
+const tap = f => x => {
+  f(x);
+  return x;
+};
+
+/** Logs the given label and a provided object to the console, the returns the object.
+ * @function trace
+ * @param {String} label
+ * @returns {Function}
+ */
+const trace = label => tap(console.log.bind(console, label + ':' || ''));
+
+/**
+ * Divides a number by 100.
+ * @function divideBy100
+ * @param {Number} num
+ * @returns {Number} 
+ */
+const divideBy100 = num => num / 100;
+
+/**
+ * Returns a number to 2 decimal places.
+ * @function roundTo2dp
+ * @param {Number} num
+ * @returns {String}
+ */
+const roundTo2dp = num => num.toFixed(2);
+
+/**
+ * Adds a dollar sign in front of a number or string.
+ * @function addDollarSign
+ * @param {Number|String} str
+ * @returns {String}
+ */
+const addDollarSign = str => '$' + String(str);
+
+/**
+ * Add commas every three digits.
+ * @function addSeparators
+ * @param {Number|String} str
+ * @returns {String}
+ */
+const addSeparators = str => {
+  // Insert commas before the decimal point
+  str = str.replace(/(?<!\.\d+)\B(?=(\d{3})+\b)/g, `,`);
+  // Insert commas after the decimal point
+  str = str.replace(/(?<=\.(\d{3})+)\B/g, `,`);
+  return str;
+}
+
+/**
+ * Convert cents to dollars.
+ * @function centsToDollars
+ * @param {Number} num - The value in cents.
+ * @returns {String} - The value in dollars, with a currency symbol and separators.
+ */
+const centsToDollars = compose(
+  trace('addSeparators'),
+  addSeparators,
+  trace('addDollarSign'),
+  addDollarSign,
+  trace('roundTo2dp'),
+  roundTo2dp,
+  trace('divideBy100'),
+  divideBy100,
+  trace('argument'),
+);
+
+centsToDollars(100000000);