reduce方法在没有给出初始值的情况下将累加器设置为数组的第一项

reduce 可以递归定义,例如:

// <Except some details to do with empty slots>
Array.prototype.reduce = function(f, initial_value) {
    if (this.length === 0) {
        if (arguments.length > 1) {
            return initial_value;
        }
        throw TypeError('reduce of empty array with no initial value');
    }
    if (arguments.length > 1) {
        // Starting value given.
        const copy = this.slice();
        const first = copy.shift();
        return copy.reduce(f, f(initial_value, first));
    } else {
        // No starting value given
        // What should happen here?
        const copy = this.slice();
        const first = copy.shift();
        // return copy.reduce(f, f(intial_value, first));

        // Nothing was given as the initial_value.
        // What is the same as "nothing"?
        // Applying the function with an `identity` value
        // so that `f(identity, first) === first` means
        // that this `identity` is the same as nothing.

        // Of course, we can't know what that is if there
        // are no elements in the list, so we threw an error before.
        return copy.reduce(f, first);
    }
};

一般来说 减少 (a, b) => a + b ,那将是 0 . 为了 (a, b) => a * b ,可能是 1 . 为了 (a, b) => Math.max(a, b) ,可能是 -Infinity

对于你的功能,正如你正确地写的, [] .

一般来说,初始值应该是 identity . 所以如果你不违约, 减少

在您的例子中,它稍微复杂一点,因为如果您定义“equality”,那么函数只有一个“identity” (Input) x === (Output) [x] .

见 the reduce function 更多信息。

扩散算子( ...memo )将第一个值转换为字符数组。从 MDN

从索引1开始的回调函数,跳过第一个索引。如果 提供了initialValue,它将从索引0开始。

const output = Object.keys({
  name: "david",
  age: 30,
  shoesize: 45
}).reduce((memo, key, i) => (i < 2 ? [memo] : memo).concat(key));
//                           ^ so convert the first value to array
console.log(output);