Detailed explanation of several methods of dimensionality reduction of JS arrays

let arr=[1,2,[3,4],5];
let arr1=[].concat(...arr);
//First use the extension operator to break up the outermost array. Concat has the function of breaking and glueing the array(arr1);
// [1, 2, 3, 4, 5]

//Compare writing method of using apply to break up data before ES6var arr2=[1,2,[3,4],5];
var arr3=[].([],arr2);
(arr3);
// [1, 2, 3, 4, 5]

// Several ways to use array functions to reduce the dimension recursively//some&concat
//Use some to check whether the array contains an array and call it to reduce the dimensions yourselffunction fun(arr){
  arr=[].concat(...arr); 
  // Check whether the array after dimensionality reduction still contains subarrays  let hasArray=(function(elem){
      return (elem);
  })
  if(hasArray){ //If subarrays are included      arr=fun(arr);//You can only reduce the dimension once again until the check does not contain subarrays anymore  }
    return arr;
};

//forEach&instanceof
//Judge each element of the array to see if it is the array that continues to call itself, and does not put it into an empty array prepared in advancefunction fun2(arr){
  let ret = [];
  let toArr = function(arr){
    (function(item){
      item instanceof Array ? toArr(item) : (item);
    });
  }
  toArr(arr);
  return ret;
}

//reduce&concat
//Finally prepare an empty array to see if each item passed into the array is an array. Then call it again. If you use concat to summarize it into the empty array.function fun3(sarr){
    return ((pre,val)=>{
        return (val) ? (fun3(val)): (val)
    },[])
}

var arr1 = [1, 2, [3, 4]];
(); 
// [1, 2, 3, 4]
 
var arr2 = [1, 2, [3, 4, [5, 6]]];
();
// [1, 2, 3, 4, [5, 6]]
 
var arr3 = [1, 2, [3, 4, [5, 6]]];
(2);
// [1, 2, 3, 4, 5, 6]
 
//Use Infinity as depth to expand nested arrays of any depth(Infinity); 
// [1, 2, 3, 4, 5, 6]