use let instead of var to define loop variables

[MichaelKreil]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

Use "let" instead of "var" to define loop variables.

Alternate Designs

I can't see any other approaches.

Benefits

Defining loop variables with "var" can lead to unwanted side effects. Therefore it is recommended to use "let".

Possible Drawbacks

There might be some developers who are not using ES6, like very old legacy versions of node.js. I believe node.js uses "let" since version 4.0.

Applicable Issues

At some point, we will have to switch to "let". The question is: when, if not now?

[icecream17]

Actually I redact that approval since there's one that also uses const, see #691