fetch-sharepoint-post-newitem.js

var getHeaders = new Headers({
	'X-RequestDigest': document.getElementById('__REQUESTDIGEST').value,
	'Accept': 'application/json; odata=verbose'
})

var getOptions = { 
	method: 'GET',
	headers: getHeaders,
	credentials: 'include'
}

fetch(_spPageContextInfo.webAbsoluteUrl + "/_api/lists/getbytitle('Sample List')?$select=ListItemEntityTypeFullName", getOptions).then(function(response) {
	if (response.ok) {
		return response.json();
	}
	else {
		throw new Error('Network response was not ok.');
	}
}).then(function(data) {
	// Our new list item metadata
	var metadata = {
		Title: 'New List Item',
		__metadata:  {
			type: data.d.ListItemEntityTypeFullName
		}
	}
	
	// POST request headers
	var postHeaders = new Headers({
		'X-RequestDigest': document.getElementById('__REQUESTDIGEST').value,
		'Accept': 'application/json; odata=verbose',
		'Content-Type': 'application/json; odata=verbose'
	});
	
	// POST request options
	var postOptions = {
		method: 'POST',
		headers: postHeaders,
		credentials: 'include',
		body: JSON.stringify(metadata)
	}
	
	// Return the fetch result so we can continue chaining function calls
	return fetch(_spPageContextInfo.webAbsoluteUrl + "/_api/lists/getbytitle('Sample List')/items", postOptions);
	
}).then(function(response) {
	if (response.ok) {
		// we all good
		console.log('Response was ok!')
	}
	else {
		throw new Error('Network response was not ok.');
	}
}).catch(function(error) {
	// Error handling code goes here
	console.error(error);
	alert('Error has been logged to the console');
})