SOURCE

//使用强制类型转换
function myFlatter1(arr=[]) {
    if(typeof arr[0] === 'number'){
        return arr.toLocaleString().split(',').map((item)=>Number(item));
    }else{
        return arr.toLocaleString().split(',');
    }   
}

//使用JSON.striify.parse加正则去[]
function myFlatter2(arr=[]) {
    let result = JSON.stringify(arr);
    result = result.replace(/(\[|\])/g,'');
    result = '[' + result + ']';
    return JSON.parse(result);

}

//使用while循环+findeIndex+展开运算符...
function myFlatter3(arr=[]){  
    while(arr.findIndex((item)=>Array.isArray(item))>0){
        arr = [].concat(...arr);
    }
    return arr;
}

//使用reduce()+递归
function myFlatter4(arr = []){
    return arr.reduce((pre,current, currentIndex, arr)=>{
        if(Array.isArray(current)){
            return pre.concat(myFlatter4(current));
        }else{
            return pre.concat(current);
        }
    },  []);
}
const arr1 = [1,2,[3,4,4],5];
const arr2 = ['果',['子',['我','爱','你'],'极'],'了'];
console.log(myFlatter1(arr1));
console.log(myFlatter1(arr2));

console.log(myFlatter2(arr1));
console.log(myFlatter2(arr2));

console.log(myFlatter3(arr1));
console.log(myFlatter3(arr2));

console.log(myFlatter4(arr1));
console.log(myFlatter4(arr2));