Shorthand anonymous functions

JS allows you to create shorthand anonymous functions, but it gets confused if you're trying to return an Object.

(a,b) => a+b
(a,b) => {sum: a+b}  // cannot return an Object like this, because curlies mean body
(a,b) => { let sum = a+b; return sum; }

Constants are mutable

JS constants prevent reassignment, but allow inner mutation. (i.e. like constant pointers to non-constant values in C).

const a = [];
a.push(1); // allowed
a = [];    // not allowed

Circular Dependency Traps

// inside app/main/index.js
import Foo from 'app/main/foo';
import Bar from 'app/main/bar';

export default {
  Foo,
  Bar,
};

// inside app/main/foo.js
import {Bar} from 'app/main';   // Circular Dependency will silently null all of `app/main`
import Bar from 'app/main/bar'; // <--- workaround that won't fail

// ...