将对象键值从函数推入数组

Question

在我当前的代码中，我没有得到所需的输出，因为键 obj[2] 的值更新为 2.4，因为该值是数字而不是数组。

有没有一种简单的方法可以将属性值存储为数组并将这些元素推送到数组中？（见代码说明）

// Create a function groupBy that accepts an array and a callback, and returns an object. groupBy will iterate through the array and perform the callback on each element. 
// Each return value from the callback will be saved as a key on the object. 
// The value associated with each key will be an array consisting of all the elements 
//that resulted in that return value when passed into the callback.

function groupBy(array, callback) {
  const obj = {};

  array.forEach((el) => {
    const evaluated = callback(el);
    obj[evaluated] = el

  });
  return obj
}
//current output : {1: 1.3, 2: 2.4}

const decimals = [1.3, 2.1, 2.4];
const floored = function(num) {
  return Math.floor(num);
};
console.log(groupBy(decimals, floored)); // should log: { 1: [1.3], 2: [2.1, 2.4] }

Answer 1

如果 undefined 为空数组，则将 obj[evaluated] 初始化为一个空数组，并将项目推入数组。

如果支持，您可以使用 Logical nullish assignment (??=) 将空数组分配给 obj[evaluated]，如果它是 null 或 undefined:

function groupBy(array, callback) {
  const obj = {};

  array.forEach((el) => {
    const evaluated = callback(el);
    (obj[evaluated] ??= []).push(el);
  });
  
  return obj
}

const decimals = [1.3, 2.1, 2.4];
const floored = function(num) {
  return Math.floor(num);
};
console.log(groupBy(decimals, floored)); // should log: { 1: [1.3], 2: [2.1, 2.4] }

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