JavaScript 减少数组以在对象中查找匹配项

Question

I am trying to incorporate array method: reduce. Basically, what I am trying to accomplish here is to reduce the array below to an object where anything that matches obj's key value.

const arr = ['a', 'c', 'e'];
const obj = { a: 1, b: 2, c: 3, d: 4 };

let output = select(arr, obj);
console.log(output); // --> { a: 1, c: 3 }

My select method:

function select(arr, obj) {
  let newObj = {};
  for (let prop in obj) {
    for (let i = 0; i < arr.length; i++) {
      if (prop === arr[i]) {
        newObj[prop] = obj[prop];
      }
    }
  }
  return newObj;
}

I set {} as initializer for arr.reduce as such if current value of array matches key of object then it would add to accumulator the key value, but I am receiving an error message from the console that if expression cannot return boolean.

Here is my attempt using .reduce() :

function select(arr, obj) {
  let result = arr.reduce(function(x, y) {
    if (y in obj) {
      x[y] = obj[y]
      return x;
    }
  }, {}) 
  return result;
}

Please advise.

Answer 1

You must always return the accumulator. Here is how to use reduce

 function select(arr, obj) { return arr.reduce(function (acc, key) { if (key in obj) acc[key] = obj[key]; return acc; }, {}); } const arr = ['a', 'c', 'e']; const obj = { a: 1, b: 2, c: 3, d: 4 }; let output = select(arr, obj); console.log(output); // --> { a: 1, c: 3 }

Answer 2

The accumulator should be returned in all the cases. I used an implementation using a filter for your reference:

const arr = ['a', 'c', 'e'];
const obj = { a: 1, b: 2, c: 3, d: 4 };

function select (obj,arr){
    let newObj = Object.keys(obj).filter(key => arr.includes(key)).reduce((acc,key) => {
                    acc[key]=obj[key]
                    return acc 
                },{})
    return newObj
}
console.log(select(obj,arr)); 

Answer 3

function select(arr, obj) {
    return arr.reduce((acc, curr) => {
        if(obj[curr]) {
            acc[curr] = obj[curr];
        }
        return acc;
    }, {})
}