Practical difference undefined vs null in JS

body = await sendEmail({
      from: 'Demos.Guru <>',
      replyTo: submission.reviewerEmail || undefined,

Today I learned one of the practical differences between null and undefined thanks to my good friend and Kickass Partner Richard Kaufman

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.