Array.prototype.reduce = function(callback, initialValue) { let acc = initialValue; for (let index = 0; index < this.length; index++) { if (!index && acc === undefined) { acc = this[index]; } else { acc = callback(acc, this[index], index, this); } } return acc; }; const arr = [3, 4, 5, 0, -1]; console.log(arr.reduce((acc, curr) => acc + curr)); // 11
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