JavaScript查找数组中指定元素的索引的方法小结

const arr = ['apple', 'banana', 'orange', 'banana'];

// 查找第一个匹配项的索引
console.log(arr.indexOf('banana')); // 1

// 查找不存在的元素
console.log(arr.indexOf('grape')); // -1

// 从指定位置开始查找
console.log(arr.indexOf('banana', 2)); // 3

const arr = ['apple', 'banana', 'orange', 'banana'];

console.log(arr.lastIndexOf('banana')); // 3
console.log(arr.lastIndexOf('banana', 2)); // 1

const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Charlie' }
];

// 查找第一个满足条件的元素索引
const index = users.findIndex(user => user.name === 'Bob');
console.log(index); // 1

// 查找年龄大于25的用户索引
const ages = [18, 22, 30, 45];
const ageIndex = ages.findIndex(age => age > 25);
console.log(ageIndex); // 2

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

// 从后往前查找第一个满足条件的元素索引
const lastIndex = numbers.findLastIndex(num => num === 3);
console.log(lastIndex); // 4

function findIndexCustom(arr, value) {
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] === value) {
      return i;
    }
  }
  return -1;
}

const arr = ['a', 'b', 'c'];
console.log(findIndexCustom(arr, 'b')); // 1
console.log(findIndexCustom(arr, 'd')); // -1

// 查找对象数组中的特定元素
const products = [
  { id: 1, name: 'Laptop', price: 1000 },
  { id: 2, name: 'Phone', price: 500 },
  { id: 3, name: 'Tablet', price: 300 }
];

// 查找价格低于400的产品索引
const cheapProductIndex = products.findIndex(product => product.price < 400);
console.log(cheapProductIndex); // 2

// 查找所有匹配项的索引
function findAllIndexes(arr, value) {
  const indexes = [];
  let currentIndex = -1;
  
  while ((currentIndex = arr.indexOf(value, currentIndex + 1)) !== -1) {
    indexes.push(currentIndex);
  }
  
  return indexes;
}

const fruits = ['apple', 'banana', 'apple', 'orange', 'apple'];
console.log(findAllIndexes(fruits, 'apple')); // [0, 2, 4]