How does the sum(1)(2)(3) function know when to stop?
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.
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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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