这道 JS 题目最快的办法是什么？

[hoythan]

use case1:
Input:['aa','bb','cc'],['dd','ee']
Output:["aadd", "aaee", "bbdd", "bbee", "ccdd", "ccee"]

use case2:
Input:['aa','bb','cc'],['dd','ee'],['ff','gg','hh']
Output:["aaddff", "aaddgg", "aaddhh", "aaeeff", "aaeegg", "aaeehh", "bbddff", "bbddgg", "bbddhh", "bbeeff", "bbeegg", "bbeehh", "ccddff", "ccddgg", "ccddhh", "cceeff", "cceegg", "cceehh"]

输入参数不定

方式 1 大概 250 - 300ms:

var mergeArr = (...args) => {
	
	const mergeString = (a,b) => {
		var s = '';

		for (var i = 0 , len = b.length; i < len; i++){
			if(i !== 0) s += ',';
			s += a.join(b[i] + ',') + b[i];
		}
		
		var arr = s.split(',');
		return arr;
	}

	var arrs = Array.apply(null, args);

	var arrData = arrs[0];
	for (var i = 1,len = arrs.length ; i < len; i++)
		arrData = mergeString(arrData,arrs[i]);

	// console.log(arrData);
};

方式 2 180 - 250 ms

function all() {
    var pro = ''
    for (var i = 0; i < arguments.length; i++) {
        var obj = JSON.stringify(arguments[i]);
        if(i !== arguments.length-1){
            pro = pro + `
                var outArr = []
                function p${i}(str) {
                    for (var i = 0; i < ${obj}.length; i++) {
                        var obj = str + ${obj}[i];
                        p${i+1}(obj)
                    }
                };`
        }else{
            pro = pro + `
                function p${i}(str) {
                    for (var i = 0; i < ${obj}.length; i++) {
                        var obj = str + ${obj}[i];
                        outArr.push(obj)
                    }
                };
                p0('')
                console.log(outArr)
                `
            eval(pro)

        }
    }
}

测试数据：

var time = new Date().getTime();
// mergeArr(['aa','bb','cc'],['dd'],['ee','ff']);
mergeArr(['aa','bb','cc'],['dd','ee'],['ff','gg','hh'],['dd','ee'],['ff','gg','hh'],['dd','ee'],['ff','gg','hh'],['dd','ee'],['ff','gg','hh'],['dd','ee'],['ff','gg','hh'],['ff','gg','hh'],['dd','ee'],['ff','gg','hh']);
console.log(new Date().getTime() - time);

还有更好的办法吗？

[hoythan]

@xiangyuecn chrome 调试工具自带数据结果缓存的。

[hoythan]

@xiangyuecn 我这边跑起来 150 - 200 左右

[xiangyuecn]

@hoythan 原来如此，怪不得以前测试速度经常飘来飘去，我去耍耍研究一下

[glacer]

笛卡尔积，MySQL 的 Nested Loop Join 了解一下

[whileFalse]

@hoythan https://gist.github.com/Just4test/47fae6e447bbc1e8dec00f3dbf60f3df

用 nodejs 8.11.2 跑过。

[baelish]

```javascript
function mergeArr(...args) {
return args.reduceRight((sum, items, i)=>{
let tt = [];
items.forEach(a=>{
sum.forEach(b=>{
tt.push(a+""+b);
})
})
return tt;
}, [""]);
}
```

[RqPS6rhmP3Nyn3Tm]

用 C 的指针写，然后上 WebAssembly

[Exin]

我来一个简短好读一点的：

```js
function mergeArr(...groups) {
const results = []
~(function merge(prefix, nextGroup, ...groups) {
if (nextGroup) nextGroup.forEach(word => merge(prefix + word, ...groups))
else results.push(prefix)
})('', ...groups)
return results
}
```

性能比楼主 append 的那一版略低一点点，但如果把 forEach 换成 for 就几乎一样了

[yichinzhu]

https://pastebin.com/GdU8EHNf
应该不慢

[hoythan]

@yichinzhu 100 - 150 左右我这里