Blocking the loop — interactive event loop example
1setTimeout(() => {2 console.log('am I late?');3}, 0);4 5console.log('start spinning');6 7while (true) {8 // spinning forever…9}A 0ms setTimeout is registered. The callback will be ready in the task queue almost immediately.
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Call stack
script
Web APIs — browser land
timeout cb 0ms
The event loop — is the call stack empty? → drain the microtask queue first → then take one callback.
Microtask queue — priority
Callback queue
Console
Fig. — Blocking the loop · step 1 of 6
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