What happens if there is a syntax error in JavaScript?
Parsing fails, no AST is built, and the entire script (or module) is not executed. The engine reports a SyntaxError and stops before any code runs.
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More FAQs in How JavaScript Executes Code Step by Step
Parsing (source to AST), compilation (AST to bytecode and machine code), execution context creation, synchronous code execution, async scheduling via Web APIs, and event loop draining queues.
Synchronous code runs first on the call stack. Then the event loop drains the microtask queue (promises), then the macrotask queue (timers, events). This is why promise callbacks run before setTimeout callbacks.
It hands the timer to a Web API, continues running synchronous code, and when the timer fires the callback is pushed to the macrotask queue. The event loop runs it only after the stack is empty.
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