js 更改箭头函数写法就报错？

Question

functionunique(arr){returnarr.reduce((prev,cur)=>prev.includes(cur)?prev:[...prev,cur],[]);}vararr=[1,1,'true','true',true,true,15,15,false,false,undefined,undefined,null,null,NaN,NaN,'NaN',0,0,'a','a',{},{}];console.log(unique(arr));//正常打印functionunique(arr){returnarr.reduce((prev,cur)=>{returnprev.includes(cur)?prev:[...prev,cur],[];});}vararr=[1,1,'true','true',true,true,15,15,false,false,undefined,undefined,null,null,NaN,NaN,'NaN',0,0,'a','a',{},{}];console.log(unique(arr));//错报问题描述更改了箭头函数的写法为什么就会报错？UncaughtTypeError:prev.includesisnotafunction

Answer 1

reduce的第二个参数没传
functionunique(arr){
returnarr.reduce((prev,cur)=>{
returnprev.includes(cur)?prev:[...prev,cur];
},[]);
}
vararr=[1,1,'true','true',true,true,15,15,false,false,undefined,undefined,null,null,NaN,NaN,'NaN',0,0,'a','a',{},{}];
console.log(unique(arr));
                            

Answer 2

functionunique(arr){
returnarr.reduce((prev,cur)=>{
returnprev.includes(cur)?prev:[...prev,cur];
},[]);
}
vararr=[1,1,'true','true',true,true,15,15,false,false,undefined,undefined,null,null,NaN,NaN,'NaN',0,0,'a','a',{},{}];
console.log(unique(arr));