javascript - 为什么 `array.reduce()` 从索引 1 开始

Question

我想知道为什么 array.reduce() 中的索引在下面的示例中从 1 而不是 0 开始

([11,22,33,44]).reduce((acc, val, index) => console.log(val));
//This outputs 22, 33 and 44 and skips 11

Answer 1

如果您没有将值作为第二个参数传递，则累加器将采用第一个值:

// add a vlaue to start
([11,22,33,44]).reduce((acc, val, index) => console.log(val), 0);
// now all values are iterated

如果您记录累加器，您可以看到所有值在没有第二个参数的情况下是如何使用的:

// Show accumulator return value
let final = ([11,22,33,44]).reduce((acc, val, index) => (console.log("acc:", acc, "val:", val), val));

// final is the last object that would have been the accumulator
console.log("final:", final)