Code Completion 5 Challenges

Complete the code: Data Types & the CORS Trap

Fill in the missing payload types that keep the derived Content-Type on the safelist.

Beginner ~4 min read

Send JSON without triggering Chrome's Blob throw or a preflight.

js Your answer

// safelisted, no-cors, no preflight:
navigator.sendBeacon(url, );
// server: JSON.parse(await req.text())

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const body = new ({ event: 'pageview', id: 42 });
navigator.sendBeacon(url, body);

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// switches the request to cors mode + preflight:
new Blob([json], { type: '' })

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// derives a NON-safelisted Content-Type -> cors mode -> preflight
const payload = ;

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// TypeError: length cannot be determined at queueing time
const stream = new ();
fetch(url, { method: 'POST', body: stream, keepalive: true });

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