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Avoid xhr request to example.com if fetch is supported #65

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Jan 12, 2017
Merged

Avoid xhr request to example.com if fetch is supported #65

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43 changes: 31 additions & 12 deletions lib/capability.js
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Original file line number Diff line number Diff line change
Expand Up @@ -6,21 +6,33 @@ try {
exports.blobConstructor = true
} catch (e) {}

// Service workers don't have XHR
var xhr = null
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) {
// 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
Expand All @@ -34,13 +46,20 @@ function checkTypeSupport (type) {
var haveArrayBuffer = typeof global.ArrayBuffer !== 'undefined'
var haveSlice = haveArrayBuffer && isFunction(global.ArrayBuffer.prototype.slice)

exports.arraybuffer = haveArrayBuffer && checkTypeSupport('arraybuffer')
// 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')
exports.overrideMimeType = xhr ? isFunction(xhr.overrideMimeType) : false

// 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) {
Expand Down