ericelliott

const doubleMap = numbers => numbers.map(n => n * 2);

console.log(doubleMap([2, 3, 4])); // [4, 6, 8]

ericelliott

const doubleMap = numbers => {
  const doubled = [];
  for (let i = 0; i < numbers.length; i++) {
    doubled.push(numbers[i] * 2);
  }
  return doubled;
};

console.log(doubleMap([2, 3, 4])); // [4, 6, 8]

ericelliott

const a = Object.freeze({
  foo: { greeting: 'Hello' },
  bar: 'world',
  baz: '!'
});

a.foo.greeting = 'Goodbye';

console.log(`${ a.foo.greeting }, ${ a.bar }${a.baz}`);

ericelliott

const a = Object.freeze({
  foo: 'Hello',
  bar: 'world',
  baz: '!'
});

a.foo = 'Goodbye';
// Error: Cannot assign to read only property 'foo' of object Object

ericelliott

const x = {
  val: 2
};

const x1 = x => Object.assign({}, x, { val: x.val + 1});

const x2 = x => Object.assign({}, x, { val: x.val * 2});

console.log(x1(x2(x)).val); // 5

ericelliott

// With shared state, the order in which function calls are made
// changes the result of the function calls.
const x = {
  val: 2
};

const x1 = () => x.val += 1;

const x2 = () => x.val *= 2;

ericelliott

const doSomething = (err) => new Promise((resolve, reject) => {
  if (err) return reject(err);

  setTimeout(() => {
    resolve(42);
  }, 1000);
});

const log = value => console.log(value);

ericelliott

import { Observable } from 'rxjs/Observable';
// then patch import only needed operators:
import 'rxjs/add/operator/map';
import 'rxjs/add/observable/from';

const foo = Observable.from([1, 2, 3]);
foo.map(x => x * 2).subscribe(n => console.log(n));

ericelliott

const check = fn => (params = []) => {
  const { required } = fn;
  const missing = required.filter(param => !(param in params));

  if (missing.length) {
    throw new Error(`${ fn.name }() Missing required parameter(s):
    ${ missing.join(', ') }`);
  }

  return fn(params);

ericelliott

const fn = (fn, {required = []}) => (params = {}) => {
  const missing = required.filter(param => !(param in params));

  if (missing.length) {
    throw new Error(`${ fn.name }() Missing required parameter(s):
    ${ missing.join(', ') }`);
  }

  return fn(params);
};