Answer
beginWork calculates changes and creates or updates child fibers, while completeWork builds DOM nodes or side-effect lists. This separation allows React to pause between units of work for better concurrency.
Quick Answer
Begin work processes a fiber top-down — determining what children it renders, creating child fibers, reconciling with previous renders. Complete work processes a fiber bottom-up — collecting effects, building the updated props, and assembling the result. This two-pass structure lets React walk the tree once without recursion.
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SugharaIQ Editorial Team
Verified Answer
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Source: SugharaIQ