Practical difference undefined vs null in JS

```js
body = await sendEmail({
      from: 'Demos.Guru <support@demos.guru>',
      replyTo: submission.reviewerEmail || undefined,
      to: user.email,
```
Today I learned one of the practical differences between `null` and `undefined` thanks to my good friend and Kickass Partner Richard Kaufman <https://richardkaufman.xyz>.

In the code snippet shown, setting `replyTo` to `undefined` makes it disappear from the object, whereas setting it to `null` would leave the property in the object, with its value set to `null`. Makes perfect sense.

obie-fernandez
February 12, 2019