SOURCE

// 数组去重1:filter+indexOf
function uniqueArr(arr = []){
    return arr.filter((item,index,array)=>{
        return index === array.indexOf(item)
    })
}

// 数组去重2:reduce+includes
function uniqueArr(arr = []){
    return arr.reduce((pre,cur)=>{
        return pre.includes(cur) ? pre: pre.concat(cur)
    },[])
}

// 数组去重3:forEach+indexOf
function uniqueArr(arr = []){
    const newArr = []
    arr.forEach(item=>{
        if(!~newArr.indexOf(item)){
             newArr.push(item)
        }
    })
    return newArr
}

// 数组去重4:new Set()
function uniqueArr(arr = []){
    return [...new Set(arr)]
}
const arr = uniqueArr([1,2,3,2,1])
console.log(arr)