V8

V8 was designed to be multi-tenant. It was designed to run the code from the many tabs in the browser in isolated environments and within a single process.

Node.js and similar runtimes were not designed for multi-tenant

Isolates

Isolates are lightweight contexts that group variables with the code allowed to mutate them. A single process can run thousands of Isolates, seamlessly switching between them.

Isolates were designed to start quickly and to not allow one Isolate to access the memory of another.

JavaScript runtime overhead is paid once. Then, Isolates allow to run scripts with almost no individual overhead.

Isolates can start around a hundred times faster than a Node.js process on a local machine. Isolates consume an order of magnitude less memory than that Node.js process.

Isolates start in 5 milliseconds.

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