title: Can we use `const` instead of `let` everywhere?
body: <p>With <code>flowtype</code> we prefer <code>const</code> against <code>let</code></p>
  
  <p>I have function that needs to be worked in most performant way and it's works well for me, it's compare two arrays, so it's cool example for my question:</p>
  
  <pre><code>/**
  * @function compare
  * @description function compares two arrays with ids to figure out: are they equal? (elements position can be different)
  * @param arraFrom {Array}
  * @param arraTo {Array}
  * @param compareFunction {Function}
  * @returns {Boolean}
  */
  function compare(arraFrom, arraTo, compareFunction) {
    let notEqual = true;
    if(arraFrom.length !== arraTo.length) return false;
      for (let i = 0; i &lt; arraFrom.length; i++) {
        notEqual = true;
        for (let j = 0; j &lt; arraTo.length; j++) {
          if (compareFunction ?
              compareFunction(arraFrom[i], arraTo[j]) :
            arraFrom[i] === arraTo[j]) {
            notEqual = false;
            break;
          }
        }
        if (notEqual) return false;
     }
     return true;
  }
  </code></pre>
  
  <p>Question is: how can we implement it without using <code>let</code> in the most performant way?</p>
  
  <p>Thank you!</p>