# entries 和 fromEntries 用法实践

# 1、获取指定属性

const obj = {
  test1: 1,
  test2: 2,
  test3: 3,
  test4: 4,
};

// 获取指定属性的对象
function getKeys1(obj, ...keys) {
  const temp = Object.entries(obj).filter(([key]) => keys.includes(key));
  return Object.fromEntries(temp);
}

console.log(getKeys1(obj, "test4", "test2", "test6"));
// { test2: 2, test4: 4 }

// 获取指定属性顺序的对象
function getKeys2(obj, ...keys) {
  const temp = Object.entries(obj);
  const tmp = [];
  keys.forEach((i) => {
    const finds = temp.find(([key]) => key === i);
    finds && tmp.push(finds);
  });
  return Object.fromEntries(tmp);
}

console.log(getKeys2(obj, "test4", "test2", "test6"));
// { test4: 4, test2: 2 }

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# 2、属性和值对调

// 属性和值对调
function getValueKey(obj) {
  const temp = Object.entries(obj).map(([key, value]) => [value, key]);
  return Object.fromEntries(temp);
}

console.log(getValueKey(obj));
// { '1': 'test1', '2': 'test2', '3': 'test3', '4': 'test4' }

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# 3、自实现

// 实现entries方法
function myEntries(obj) {
  const temp = [];
  for (const key in obj) {
    temp.push([key, obj[key]]);
  }
  return temp;
}

console.log(myEntries(obj));
// [ [ 'test1', 1 ], [ 'test2', 2 ], [ 'test3', 3 ], [ 'test4', 4 ] ]

// 实现fromEntries方法
function myFromEntries(list) {
  const obj = {};
  for (const [key, value] of list) {
    let keyName;
    if (typeof key === "string" || typeof key === "symbol") {
      keyName = key;
    } else {
      keyName = String(key);
    }
    obj[keyName] = value;
  }
  return obj;
}

const list = [
  ["test1", 1],
  ["test2", 2],
  ["test3", 3],
  ["test4", 4],
];
console.log(myFromEntries(list));
// { test1: 1, test2: 2, test3: 3, test4: 4 }

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# 4、总结

获取指定属性：通过配合 filter 和 includes 方法来实现
属性和值对调：通过配合 map 方法来实现
实现 entries 方法：通过 for...in 来遍历对象
实现 fromEntries 方法：通过 for...of 来遍历数组，再进行组装

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