javascript - 如何统计每种类型的实例数？

Question

const data = ['soccer', 'soccer', 'basketball', 'basketball', 'badminton', 'swimming', 'soccer', 'squash', 'badminton', 'swimming', 'soccer', 'squash', 'soccer', 'basketball', 'fencing'];

const sports = data.reduce(function(obj, item) {
  if (!obj[item]) //to create an object if it doesn't exist
    obj[item] = 0;
  else
    obj[item]++; //to increment the object count if it exist
  return obj;
}, {});

console.log(sports);

我收到的输出:

Object { soccer: 0, basketball: 0, badminton: 0, swimming: 0, squash: 0, fencing: 0 }

我想要的输出:

Object { soccer: 5, basketball: 3, badminton: 2, swimming: 2, squash: 2, fencing: 1 }

出了什么问题？

Answer 1

在 JavaScript 中，零 evaluates to false .
所以 !obj[item] 是 true 当 obj[item] == 0 并且什么都不会递增。

const obj = {
  'soccer': 0
};

// true
console.log(!obj['basketball']);

// also true!
console.log(!obj['soccer']);

一个解决方案是将新属性初始化为 1 而不是 0。无论如何，任何值都不能为零，因为每个属性在数据集中至少存在一次，这可以避免将零与未定义混淆。

const data = ['soccer', 'soccer', 'basketball', 'basketball', 'badminton', 'swimming', 'soccer', 'squash', 'badminton', 'swimming', 'soccer', 'squash', 'soccer', 'basketball', 'fencing'];

const sports = data.reduce(function(obj, item) {
  if (!obj[item])
    obj[item] = 1; // initialize property to 1 if it is falsy
  else
    obj[item]++; // otherwise increment the value
  return obj;
}, {});

console.log(sports);

但在我看来，else 是不必要的，Andy's method有点简单。

只是为了好玩，这是使用 ternary operator 的另一种方法:

const data = ['soccer', 'soccer', 'basketball', 'basketball', 'badminton', 'swimming', 'soccer', 'squash', 'badminton', 'swimming', 'soccer', 'squash', 'soccer', 'basketball', 'fencing'];

const sports = data.reduce(function(obj, item) {
  obj[item] = obj[item] ? obj[item] + 1 : 1;
  return obj;
}, {});

console.log(sports);

和another method来自 snak :

const data = ['soccer', 'soccer', 'basketball', 'basketball', 'badminton', 'swimming', 'soccer', 'squash', 'badminton', 'swimming', 'soccer', 'squash', 'soccer', 'basketball', 'fencing'];

const sports = data.reduce(function(obj, item) {
  obj[item] = (obj[item] || 0) + 1;
  return obj;
}, {});

console.log(sports);

供引用，另见:
How to increment an object property value if it exists, else set the initial value?
short javascript code for: initialize to zero or increment
Count the occurrence of each word in a phrase using javascript