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request-handler.js
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request-handler.js
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'use strict';
const AsyncStream = require('./streams/async-stream');
const ContentLengthStream = require('./streams/content-length-stream');
const { TEMPLATE_NOT_FOUND } = require('./fetch-template');
const processTemplate = require('./process-template');
const {
getLoaderScript,
getFragmentAssetsToPreload,
nextIndexGenerator
} = require('./utils');
// Events emitted by fragments on the template
const FRAGMENT_EVENTS = [
'start',
'response',
'end',
'error',
'timeout',
'fallback',
'warn'
];
/**
* Process the HTTP Request to the Tailor Middleware
*
* @param {Object} options - Options object passed to Tailor
* @param {Object} request - HTTP request stream of Middleware
* @param {Object} response - HTTP response stream of middleware
*/
module.exports = function processRequest(options, request, response) {
this.emit('start', request);
const {
fetchContext,
fetchTemplate,
parseTemplate,
filterResponseHeaders,
maxAssetLinks,
amdLoaderUrl
} = options;
const asyncStream = new AsyncStream();
asyncStream.once('plugged', () => {
asyncStream.end();
});
const contextPromise = fetchContext(request).catch(err => {
this.emit('context:error', request, err);
return {};
});
const templatePromise = fetchTemplate(request, parseTemplate);
const responseHeaders = {
// Disable cache in browsers and proxies
'Cache-Control': 'no-cache, no-store, must-revalidate',
Pragma: 'no-cache',
'Content-Type': 'text/html'
};
let shouldWriteHead = true;
const contentLengthStream = new ContentLengthStream(contentLength => {
this.emit('end', request, contentLength);
});
const handleError = err => {
this.emit('error', request, err);
if (shouldWriteHead) {
shouldWriteHead = false;
let statusCode = 500;
if (err.code === TEMPLATE_NOT_FOUND) {
statusCode = 404;
}
response.writeHead(statusCode, responseHeaders);
// To render with custom error template
if (typeof err.presentable === 'string') {
response.end(`${err.presentable}`);
} else {
response.end();
}
} else {
contentLengthStream.end();
}
};
const handlePrimaryFragment = (fragment, resultStream) => {
if (!shouldWriteHead) {
return;
}
shouldWriteHead = false;
fragment.once('response', (statusCode, headers) => {
// Map response headers
if (typeof filterResponseHeaders === 'function') {
Object.assign(
responseHeaders,
filterResponseHeaders(fragment.attributes, headers)
);
}
if (headers.location) {
responseHeaders.location = headers.location;
}
// Make resources early discoverable while processing HTML
const assetsToPreload = getFragmentAssetsToPreload(
headers,
request.headers
);
// Loader script must be preloaded before every fragment asset
const loaderScript = getLoaderScript(amdLoaderUrl, request.headers);
loaderScript !== '' && assetsToPreload.unshift(loaderScript);
responseHeaders.link = assetsToPreload.join(',');
this.emit('response', request, statusCode, responseHeaders);
response.writeHead(statusCode, responseHeaders);
resultStream.pipe(contentLengthStream).pipe(response);
});
fragment.once('fallback', err => {
this.emit('error', request, err);
response.writeHead(500, responseHeaders);
resultStream.pipe(contentLengthStream).pipe(response);
});
fragment.once('error', err => {
this.emit('error', request, err);
response.writeHead(500, responseHeaders);
response.end();
});
};
Promise.all([templatePromise, contextPromise])
.then(([parsedTemplate, context]) => {
// extendedOptions are mutated inside processTemplate
const extendedOptions = Object.assign({}, options, {
nextIndex: nextIndexGenerator(0, maxAssetLinks),
asyncStream
});
const resultStream = processTemplate(
request,
extendedOptions,
context
);
let isFragmentFound = false;
resultStream.on('fragment:found', fragment => {
isFragmentFound = true;
FRAGMENT_EVENTS.forEach(eventName => {
fragment.once(eventName, (...args) => {
const prefixedName = 'fragment:' + eventName;
this.emit(
prefixedName,
request,
fragment.attributes,
...args
);
});
});
const { primary } = fragment.attributes;
primary && handlePrimaryFragment(fragment, resultStream);
});
resultStream.once('finish', () => {
const statusCode = response.statusCode || 200;
if (shouldWriteHead) {
shouldWriteHead = false;
// Preload the loader script when atleast
// one fragment is present on the page
if (isFragmentFound) {
const loaderScript = getLoaderScript(
amdLoaderUrl,
request.headers
);
loaderScript !== '' &&
(responseHeaders.link = loaderScript);
}
this.emit('response', request, statusCode, responseHeaders);
response.writeHead(statusCode, responseHeaders);
resultStream.pipe(contentLengthStream).pipe(response);
}
});
resultStream.once('error', handleError);
parsedTemplate.forEach(item => {
resultStream.write(item);
});
resultStream.end();
})
.catch(err => {
handleError(err);
});
};