The answer
Two approaches: (1) call with no argument: sum(1)(2)(3)() - check if b !== undefined. (2) toString override: sum(1)(2)(3) + 0 - JavaScript calls toString when arithmetic is performed.
Currying Interview Questions in JavaScript
How does the sum(1)(2)(3) function know when to stop?
More FAQs in Currying Interview Questions in JavaScript
function sum(a) { return function(b) { if (b !== undefined) return sum(a + b); return a; }; }. Call with no argument to terminate and return the accumulated sum.
Same pattern: function sum(a) { return function(b) { if (b !== undefined) return sum(a + b); return a; }; }. Or use toString: function sum(n) { const fn = m => sum(n + m); fn.toString = () => n; return fn; }.
function curry(fn) { return function curried(...args) { if (args.length >= fn.length) return fn(...args); return (...next) => curried(...args, ...next); }; }.
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