为什么我的 reduce 累加器会重置？

Question

如何保留 reduce 函数的累计值？每次迭代都会重置对象值。

const a = [1, 2, 3, 4, 5];

const b = {
  1: {
    name: 'Dan',
    age: 25
  },
  2: {
    name: 'Peter',
    age: 28
  },
  3: {
    name: 'Mark',
    age: 38
  },
  4: {
    name: 'Larry',
    age: 32
  },
  5: {
    name: 'Simon',
    age: 25
  },
}

const f = a.reduce((acc, val) => {
  console.log({
    acc
  })
  return {
    [val]: {
      age: b[val].age
    }
  }
}, {})

console.log(f); // 5: {age: 25}

我想要的结果是：

{
  1: { age: 25 },
  2: { age: 28 },
  3: { age: 38 },
  4: { age: 32 },
  5: { age: 25 },
}

(本例为demo)

Answer 1

使用object spread (like this example) or Object.assign()将前一个累加器添加到返回值：

const a = [1, 2, 3, 4, 5];

const b = {"1":{"name":"Dan","age":25},"2":{"name":"Peter","age":28},"3":{"name":"Mark","age":38},"4":{"name":"Larry","age":32},"5":{"name":"Simon","age":25}};

const f = a.reduce((acc, val) => ({
  ...acc, // previous accumulator
  [val]: {
    age: b[val].age
  }
}), {})

console.log(f); // 5: {age: 25}

Answer 2

根据MDN

The reduce() method applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value.

在每次迭代中，您在 reduce() 函数中 return 新对象，并且您没有存储该累加器的先前值。所以你需要合并或用新值分配以前的值。

One way you can use Object.assign() method to get the required result.

演示版

const a = [1, 2, 3, 4, 5];

const b = {1: {name: 'Dan',age: 25},2: {name: 'Peter',age: 28},3: {name: 'Mark',age: 38},4: {name: 'Larry',age: 32},5: {name: 'Simon',age: 25}}

let result = a.reduce((acc, val) => Object.assign(acc,{[val]: {age:b[val].age}}), {});

console.log(result);

.as-console-wrapper {max-height: 100% !important;top: 0;}

second way you can do like this obje[val]=newValue and return accumulator.

演示版

const a = [1, 2, 3, 4, 5];

const b = {1: {name: 'Dan',age: 25},2: {name: 'Peter',age: 28},3: {name: 'Mark',age: 38},4: {name: 'Larry',age: 32},5: {name: 'Simon',age: 25}}

let result = a.reduce((acc, val) =>{
acc[val]= {age:b[val].age};
return acc;
}, {});

console.log(result);

.as-console-wrapper {max-height: 100% !important;top: 0;}

Another way you can combine using the spread syntax

演示版

const a = [1, 2, 3, 4, 5];

const b = {1: {name: 'Dan',age: 25},2: {name: 'Peter',age: 28},3: {name: 'Mark',age: 38},4: {name: 'Larry',age: 32},5: {name: 'Simon',age: 25}}

let result = a.reduce((acc, val) => {
return {...acc,...{[val]:{age:b[val].age}}}
}, {});

console.log(result);

.as-console-wrapper {max-height: 100% !important;top: 0;}

Answer 3

如果你的 reduce 函数中碰巧有一个 if else 语句，你的 reduce 累加器也可以重置，因此你 return 一个 if 语句的累加器但忘记处理 else 语句并且不 return 一个累加器 - 在这种情况下，当触发 else 语句时，没有累加器被 returned 并且累加器变为未定义。

例如这个有效：

let reducedArray = array.reduce((acc, curr) => {
          if (
            curr[
              property_name
            ]
          ) {
            return {
              ...acc,
              ...{
                [curr.id]:
                  (acc[curr.id] ?? 0) +
                  curr[
                    property_name
                  ]!
              },
            };
          } else {
            return acc;
          }
        }, {} as { [key: string]: number });

但这行不通：

let reducedArray = array.reduce((acc, curr) => {
          if (
            curr[
              property_name
            ]
          ) {
            return {
              ...acc,
              ...{
                [curr.id]:
                  (acc[curr.id] ?? 0) +
                  curr[
                    property_name
                  ]!
              },
            };
          }
        }, {} as { [key: string]: number });

为什么我的 reduce 累加器会重置？

Why does my reduce accumulator reset?

javascript

arrays

reduce