You may be able to use jquery:
$.getJSON('https://api.thingspeak.com/channels/' + channel_id + '/fields/1.json?api_key=' + api_key , function(data) {
if (data.feeds==....) //Do your check here
})
You may need to load the library first:
https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js