数组去重

效率：哈希表 > Set >> indexof+array >> indexof+filter

哈希表方式

function uniqueArray1(arr) {
    const map = {}
    const a = []
    for (let i = 0; i < arr.length; i++) {
        let v = arr[i]
        if (map[v]) continue
        else {
            map[v] = 1
            a.push(arr[i])
        }
    }
    return a
}

indexOf

function uniqueArray2(arr) {
    var a = []
    for (let i = 0; i < arr.length; i++) {
        if (a.indexOf(arr[i]) === -1 && arr[i] !== '') a.push(arr[i])
    }
    return a
}

indexOf + filter

function uniqueArray3(a) {
    return a.filter(function onlyUnique(value, index, self) {
        return self.indexOf(value) === index
    })
}

new Set

function uniqueArray4(a) {
    return [...new Set(a)]
}