function appendStyle(styleSrc) {
var style = document.createElement('link')
style.src = styleSrc
style.rel = 'stylesheet'
style.type = 'text/css'
document.head.appendChild(style)
}
function appendScript(scriptSrc) {
var script = document.createElement('script')
script.src = scriptSrc
document.head.appendChild(script)
}
if( window.innerWidth <= 640 ) {
appendStyle('http://example.com/mobile-style.css')
appendScript('http://example.com/mobile-script.js')
}