问题描述
我试图使功能与Array.prototype.reduce
方法相同。
function a(collection,iterator,accumulator) {
for (i of Object.keys(collection)){
if (i===0 && accumulator === undefined){
accumulator = collection[i]
continue
}
accumulator = iterator(accumulator,collection[i])
}
return accumulator;
};
a([1,2,3],function(acc,cur){return acc += cur},0) // this works fine. returns 6
a([1,cur){return acc += cur}) // expected 6 but returns NaN
a([1,function(memo){return memo}); // expected 1 but returns undefined
我不知道为什么在未传递初始值(函数a的第三个参数)的情况下,accumulator
未设置为collection[i]
并保持为undefined
的原因。
解决方法
Object.keys
始终返回由 strings 组成的数组,因此i
是一个字符串,因此i === 0
始终为false。
还要注意,您需要声明i
。目前,您的代码已成为The Horror of Implicit Globals的牺牲品。 (我建议使用严格模式,这样它们就是应该一直存在的错误。)
FWIW,这是同时解决这两个问题的最低更新(但也请参见下文):
"use strict";
function a(collection,iterator,accumulator) {
let first = true; // ***
for (const i of Object.keys(collection)){
// ^^^^^−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− *** declare `i`
if (first && accumulator === undefined){
// ^^^^^−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− *** first pass?
first = false; // ***
accumulator = collection[i]
continue
}
first = false; // ***
accumulator = iterator(accumulator,collection[i])
}
return accumulator;
};
console.log(a([1,2,3],function(acc,cur){return acc += cur},0));
console.log(a([1,cur){return acc += cur}));
console.log(a([1,function(memo){return memo}));
FWIW,我不知道您是否希望它与reduce
完全匹配,但是如果是,则您函数的逻辑与Array.prototype.reduce
的逻辑不同。有关详细信息,请参见the specification,但是reduce
会检查它得到了多少个参数,而不会检查累加器是否为undefined
。如果您为累加器明确为其指定值undefined
,它将使用该值(而不是使用第一个数组元素的值):
console.log("`undefined` for accumulator:");
["a","b","c"].reduce((acc,value) => { console.log(acc,value); return value; },undefined);
console.log("No accumulator:");
["a",value); return value; })
.as-console-wrapper {
max-height: 100% !important;
}
在减少期间,累加器也可以成为 undefined
。如果没有给出累加器,则使用的是第一个看到的值(不一定是稀疏数组中的索引0!)。 (我对上面的累加器的修复实际上将其作为副产品修复。)您的代码也使用Object.keys
,而reduce
仅使用for
循环。我没有仔细阅读,可能还会有其他差异。
"use strict";
function a(collection,...optional) { // ***
let haveAccumulator = optional.length > 0; // *** Default acc correctly
let [accumulator] = optional; // ***
for (const i of Object.keys(collection)){
// ^^^^^−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− *** declare `i`
if (!haveAccumulator) { // ***
haveAccumulator = true; // ***
accumulator = collection[i]
continue;
}
accumulator = iterator(accumulator,collection[i])
}
return accumulator;
};
console.log(a([1,function(memo){return memo}));