currying-composition-piping.js

const fs = require('fs');

const pipe = (...fns) => (x) => fns.reduce((y, f) => f(y), x);

function curry(fn) {
  return function curried(...args) {
    if (args.length >= fn.length) return fn(...args);
    return function (a) {
      return curried(...[...args, a]);
    };
  };
}

const handleError = (err) => {
  if (err) throw err;
  console.log('Saved!');
};

const datetime = {
  toString: () => new Date().toISOString(),
};

// The three functions that are the building blocks of the logging solution.
// Building block 1: formatLog
function formatLog(datetime, severity, message) {
  return `${datetime} [${severity}] - ${message}`;
}

// Building block 2: writeToConsole
function writeToConsole(message) {
  console.log(message);
  return message;
}

// Building block 3: writeToFile
function writeToFile(filePath, message) {
  fs.appendFile(filePath, message + '\n', handleError);
  return message;
}

// Calling the building blocks using nesting: no composition:
// Write a log message to the console:
writeToConsole(formatLog(new Date().toISOString(), 'INFO', 'Hello World'));

// Write a log message to a file:
writeToFile(
  '/path/to/file',
  formatLog(new Date().toISOString(), 'INFO', 'Hello World')
);

// Write a log message to both the console and a file:
writeToFile(
  '/path/to/file',
  writeToConsole(formatLog(new Date().toISOString(), 'INFO', 'Hello World'))
);

// Let's compose using currying and piping:
const curriedFormatLog = curry(formatLog);
const formatLogInfo = curriedFormatLog(datetime)("INFO");

const curriedWriteToFile = curry(writeToFile);
const writeToLogFile = curriedWriteToFile('/path/to/file');

// Pipe (or chain) some functions into a new function that does logging for us:
const logInfoToFile = pipe(formatLogInfo, writeToConsole, writeToLogFile);

// Let's do some logging:
logInfoToFile("Hello World");