Should I mention toString when solving the sum question?
Yes. After the basic solution, mention that you could also use toString or valueOf for automatic type coercion. This shows deeper JavaScript knowledge (type coercion, toString, valueOf).
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More FAQs in sum(1)(2)(3) Interview Tips for Amazon
Recognize it as currying. Start with the simple solution (terminate with empty call). Explain how closures accumulate the sum. Mention toString as an alternative. Handle follow-ups like multiple arguments.
Not returning a function (the chain breaks), not accumulating the previous value, no termination mechanism (no way to get the result), and overcomplicating (using classes or complex state instead of a simple closure).
Step by step: sum(1) returns a function with a=1. Calling it with 2 returns sum(3) (a new function with a=3). Each call accumulates. Calling with no argument returns the accumulated sum. Each step uses a closure.
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