Below is an unobtrusive way to ensure AJAX headers are set with every fetch() request, which compliments our post regarding fetch() wrappers.

const originalFetch = fetch
fetch = ((url, options) => {
	if (!options) options = {}
	Object.assign(options, { headers: { 'X-Requested-With': 'XMLHttpRequest' } })
	const promise = originalFetch(url, options).catch(error => { console.error(error) })
	return promise
})

Now every fetch() request includes the X-Requested-With: XMLHttpRequest header.

We ultimately ended up with the following alternative to avoid replacing other existing headers.

if (!options) options = {}
if (!options.headers) options.headers = {}
options.headers['X-Requested-With'] = 'XMLHttpRequest'

Fairly handy when you depend on something like CodeIgniter’s isAJAX() method.