新手求助：JS嵌套循环提取数组相似元素时重复输出数组的修复

• 问题根源在于你把打印语句写在了匹配成功的判断逻辑里：每找到一个相同元素，就会立刻打印一次当前的threeArr，所以找到90时打印[90]，找到55时打印[90, 55]，自然会输出两个数组。

• 把console.log(threeArr)移到循环外面即可，等所有匹配完成后再打印最终结果：

let onArr = [90, 66, 97, 55]
let twoArr = [90, 77, 55, 80]
let threeArr = []

for (let i = 0; i < onArr.length; i++) {
  for (let j = 0; j < twoArr.length; j++) {
    if (onArr[i] === twoArr[j]) {
      threeArr.push(onArr[i])
    }
  }
}
console.log(threeArr) // 只会打印一次最终结果：[90, 55]

• 额外优化：如果原数组存在重复元素，想避免结果数组出现重复值，可以在push前先检查元素是否已经存在于结果数组中：

let onArr = [90, 66, 97, 55, 90]
let twoArr = [90, 77, 55, 80]
let threeArr = []

for (let i = 0; i < onArr.length; i++) {
  for (let j = 0; j < twoArr.length; j++) {
    if (onArr[i] === twoArr[j] && !threeArr.includes(onArr[i])) {
      threeArr.push(onArr[i])
    }
  }
}
console.log(threeArr) // 输出 [90, 55]，不会重复添加90

• 更简洁的ES6写法：用filter和includes一行搞定，需要去重的话配合Set使用：

let onArr = [90, 66, 97, 55]
let twoArr = [90, 77, 55, 80]
// 基础版
let threeArr = onArr.filter(item => twoArr.includes(item))
// 去重版
let uniqueThreeArr = [...new Set(onArr.filter(item => twoArr.includes(item)))]
console.log(threeArr) // [90, 55]

内容的提问来源于stack exchange，提问作者user20269720