es6-feature-detection.js

var str = 'class ಠ_ಠ extends Array {constructor(j = "a", ...c) {const q = (({u: e}) => {return { [`s${c}`]: Symbol(j) };})({});super(j, q, ...c);}}' + 
          'new Promise((f) => {const a = function* (){return "\u{20BB7}".match(/./u)[0].length === 2 || true;};for (let vre of a()) {' +
          'const [uw, as, he, re] = [new Set(), new WeakSet(), new Map(), new WeakMap()];break;}f(new Proxy({}, {get: (han, h) => h in han ? han[h] ' + 
          ': "42".repeat(0o10)}));}).then(bi => new ಠ_ಠ(bi.rd));';

try {
  eval(str);
} catch(e) {
  alert('Your browser does not support ES6!')
}

/* 
 * All credits to Netflix for providing this approach to ES6 feature detection. Although this could be written in many different ways
 * this proved to be the most direct and elegant approach for me.
 * License: MIT
*/

formatted.js

class ಠ_ಠ extends Array {
  constructor(j = "a", ...c) {
    const q = (({u: e}) => {
      return { [`s${c}`]: Symbol(j) }
    })({})
    super(j, q, ...c)
  }
}

new Promise((f) => {
  const a = function* () {
    return "\u{20BB7}".match(/./u)[0].length === 2 || true
  }
  for (let vre of a()) {
    const [uw, as, he, re] = [new Set(), new WeakSet(), new Map(), new WeakMap()]
    break
  }
  f(new Proxy({}, {
    get: (han, h) => h in han ? han[h] : "42".repeat(0o10)
  }))
}).then(bi => new ಠ_ಠ(bi.rd))

Basic detection of the following features:

• Classes
• Enhanced object literals
• Template strings
• Destructuring
• Default + Rest + Spread
• let + const
• Iterators + for..of
• Generators (function* ())
• Unicode (ಠ_ಠ)
• Proxies
• Symbols
• Subclassable built-ins
• Promises
• Arrows
• New additions to APIs:
  • Math
  • Number
  • String
  • Array
  • Object