core/node_modules/stream-http/lib/capability.js

exports.fetch = isFunction(global.fetch) && isFunction(global.ReadableStream)

exports.blobConstructor = false
try {
	new Blob([new ArrayBuffer(1)])
	exports.blobConstructor = true
} catch (e) {}

// The xhr request to example.com may violate some restrictive CSP configurations,
// so if we're running in a browser that supports `fetch`, avoid calling getXHR()
// and assume support for certain features below.
var xhr
function getXHR () {
	// Cache the xhr value
	if (xhr !== undefined) return xhr

	if (global.XMLHttpRequest) {
		xhr = new global.XMLHttpRequest()
		// If XDomainRequest is available (ie only, where xhr might not work
		// cross domain), use the page location. Otherwise use example.com
		// Note: this doesn't actually make an http request.
		try {
			xhr.open('GET', global.XDomainRequest ? '/' : 'https://example.com')
		} catch(e) {
			xhr = null
		}
	} else {
		// Service workers don't have XHR
		xhr = null
	}
	return xhr
}

function checkTypeSupport (type) {
	var xhr = getXHR()
	if (!xhr) return false
	try {
		xhr.responseType = type
		return xhr.responseType === type
	} catch (e) {}
	return false
}

// For some strange reason, Safari 7.0 reports typeof global.ArrayBuffer === 'object'.
// Safari 7.1 appears to have fixed this bug.
var haveArrayBuffer = typeof global.ArrayBuffer !== 'undefined'
var haveSlice = haveArrayBuffer && isFunction(global.ArrayBuffer.prototype.slice)

// If fetch is supported, then arraybuffer will be supported too. Skip calling
// checkTypeSupport(), since that calls getXHR().
exports.arraybuffer = exports.fetch || (haveArrayBuffer && checkTypeSupport('arraybuffer'))

// These next two tests unavoidably show warnings in Chrome. Since fetch will always
// be used if it's available, just return false for these to avoid the warnings.
exports.msstream = !exports.fetch && haveSlice && checkTypeSupport('ms-stream')
exports.mozchunkedarraybuffer = !exports.fetch && haveArrayBuffer &&
	checkTypeSupport('moz-chunked-arraybuffer')

// If fetch is supported, then overrideMimeType will be supported too. Skip calling
// getXHR().
exports.overrideMimeType = exports.fetch || (getXHR() ? isFunction(getXHR().overrideMimeType) : false)

exports.vbArray = isFunction(global.VBArray)

function isFunction (value) {
	return typeof value === 'function'
}

xhr = null // Help gc