Javascript 字符串全排列组合方法实现

        let arr = []
        let combine = function (str) {
            for (let i = 0; i < str.length; i++) {
                for (let j = 0; j < str.length; j++) {
                    if (i != j) {
                        for (let k = 0; k < str.length; k++) {
                            if (i != k && j != k) {
                                // console.log(str[i]+str[j]+str[k])
                                arr.push(str[i]+str[j]+str[k])
                            }
                        }
                    }
                }
            }
        }
        combine(['a','b','ab'])
        console.log(Array.from(new Set(arr)))