Javascript assigning custom property keeps assigning indefinitely deep instead of once

I have some javascript where I’m trying to create a promotion within each of my promotion items. Right now though, for some reason it keeps on nesting itself indefinitely, and I only need it to contain one reference (see screenshot)

I’ve tried adding a check to see if promotion.promotion already exists and if it does to continue, but this just doesn’t seem to run.

What am I missing?

const promotions = [{
    title: 'My offer title',
  subtitle: 'lorem ipsum'
}, {
    title: 'My offer title 2',
  subtitle: 'lorem ipsum'
}, {
    title: 'My offer title 3',
  subtitle: 'lorem ipsum'
}]

for (const [index, promotion] of promotions.entries()) {
    if (promotion.promotion) continue
  promotion['promotion'] = promotion
}

Here’s a jsfiddle for a demo.

This is what I need for the output:

const promotions = [{
    title: 'My offer title',
  subtitle: 'lorem ipsum',
  promotion: {
    title: 'My offer title',
    subtitle: 'lorem ipsum',
  }
}, {
    title: 'My offer title 2',
  subtitle: 'lorem ipsum',
  promotion: {
    title: 'My offer title 2',
    subtitle: 'lorem ipsum',
  }
}, {
    title: 'My offer title 3',
  subtitle: 'lorem ipsum',
  promotion: {
    title: 'My offer title 3',
    subtitle: 'lorem ipsum',
  }
}]

>Solution :

for (const [index, promotion] of promotions.entries()) {
    if (promotion.promotion) continue
  promotion.promotion = { ...promotion}
}