How does V8 compile JavaScript in Node.js?
V8 parses JavaScript to an AST, compiles the AST to bytecode for the Ignition interpreter (fast startup), profiles hot code, then the TurboFan optimizing compiler generates optimized machine code for hot code. Deoptimization handles cases where assumptions break.
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More FAQs in How V8 Compiles JavaScript to Machine Code in Node.js
V8's interpreter. It compiles the AST to bytecode and executes it. Ignition starts fast, so your code runs quickly even before optimization. The interpreter also collects profiling data that drives TurboFan optimization.
V8's optimizing compiler. For hot code, TurboFan takes bytecode and profiling feedback and generates highly optimized machine code, much faster than interpreted bytecode. This is why hot code runs at near-native speed in V8.
When TurboFan's optimization assumptions break, like a function getting a different type than expected, V8 deoptimizes back to bytecode. This ensures correctness while still optimizing common cases, so the code stays correct even as types change.
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