块作用域函数: ECMAScript 6与ECMAScript 5

Question

在块作用域功能方面，ES6与ES5相比有哪些优势？我的意思是，在这两种情况下，块看起来非常相似，那么它有什么不同，性能方面，哪种方法更好呢？

ES6块

{
    function foo() {
        return 1;
    }

    foo() === 1;
    {
        function foo() {
            return 2;
        }

        foo() === 2;
    }
    foo() === 1;
}

ES5块

(function () {
    var foo = function () {
        return 1;
    }
    foo() === 1;
    (function () {
        var foo = function () {
            return 2;
        }
        foo() === 2;
    })();
    foo() === 1;
})();

Answer 1

下面是一个测试，用来展示哪个最快：

​

document.getElementById('btn').addEventListener('click', ({ target }) => {
  target.disabled = true;
  target.innerHTML = "Running…";
  const suite = new Benchmark.Suite();
  document.getElementById('ES6').style.fontWeight = '';
  document.getElementById('ES5').style.fontWeight = '';
  suite.add('ES6', () => {
    {
        function foo() {
            return 1;
        }
        foo() === 1;
        {
            function foo() {
                return 2;
            }
            foo() === 2;
        }
        foo() === 1;
    }
  }).add('ES5', () => {
    (function () {
        var foo = function () {
            return 1;
        }
        foo() === 1;
        (function () {
            var foo = function () {
                return 2;
            }
            foo() === 2;
        })();
        foo() === 1;
    })();
  })
  .on('cycle', ({target: bench}) => document.getElementById(bench.name).textContent = `${bench.name}: ~${Benchmark.formatNumber(bench.hz|0)} ops/sec (~${Benchmark.formatNumber(Math.round(1e9/bench.hz))} ns/op)`)
  .on('complete', function() {
    const el = document.getElementById(this.filter('fastest').map('name')[0]),
          hz = this.filter('fastest').map('hz')[0],
          others = this.filter('slowest').map('hz'),
          avg = others.reduce((a, b) => a + b, 0) / others.length;
    el.style.fontWeight = 'bold';
    el.textContent += ` \u{1f451} ${Math.round((1 - avg / hz) * 100, 2)}% faster`;
    target.disabled = false;
    target.innerHTML = "Run";
  })
  .run({ 'async': true });
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.2/lodash.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/benchmark/2.1.2/benchmark.min.js"></script>
<ul>
  <li id="ES6"></li>
  <li id="ES5"></li>
</ul>
<button id="btn">Run</button>

​

我电脑上的结果：

• ES6：~3,896,305次/秒(约257 ns/次) 40% faster
• ES5：~2,425,847次/秒(约412 ns/次)