JavaScript variable scope can be particularly difficult to understand and get right. The situation gets even worse when you consider the accidental creation of global variables, which is what happens when you declare a variable inside a function or the `for` clause of a for-loop without using the `let`, `const` or `var` keywords.

`let` and `const` were introduced in ECMAScript 2015, and are now the preferred keywords for variable declaration.

## Noncompliant Code Example

    function f(){
      i = 1;         // Noncompliant; i is global

      for (j = 0; j < array.length; j++) {  // Noncompliant; j is global now too
        // ...
      }
    }

## Compliant Solution

    function f(){
      var i = 1;

      for (let j = 0; j < array.length; j++) {
        // ...
      }
    }