The entries collection in observers should be non-empty

⚙ Compilation target

ES2017

⚙ Library

TypeScript

Missing / Incorrect Definition

The type in question is ResizeObserverCallback, and similar observer callbacks, like the ones in MutationObserver and IntersectionObserver. These currently accept entries in the T[] format, but it should really be [T, ...T[]]. This is because the observer callback is only ever invoked when there is at least one entry.

(I've also noticed the same issue with the ResizeObserverEntry's contentBoxSize property.)

Sample Code

const ro = new ResizeObserver(entries => {
  const entry = entries[0] // <-- This should never be undefined
  const size = entry.contentBoxSize[0] // <-- Nor this, btw
  const dim = size.blockSize
  console.log(dim / 2)
})

Documentation Link

While there is no direct verbiage that suggests the entry collection is never empty, it stems from the description of the algorithms. For example in case of the ResizeObserver:

https://drafts.csswg.org/resize-observer/#broadcast-active-resize-observations

The observer callback is never invoked if there are no observations. Otherwise, entries are created for each observation. This pretty much guarantees the entries collection is non-empty.