如何仅在循环的第一次迭代中创建一个空数组然后继续使用它？

Question

I am new to javascript . I have two functions, one called arrayToList which takes an array and produces a list from it, the other is called listToArray which takes a list and produces array from it. first one is working fine. However, in case of second one I have to create an empty array in the first iteration only of a for or while loop within which I will pass on each value of list to array I have created and at each step remove list.value and set the list to list.rest (that is rest of list) and then recursively keep calling the same function.

My problem is that I am not able to declare the array within the function. If I say if(!array) {let array=[];} then I get message that array is undefined, if I declare array as global then it is retaining values across different calls to function, which I don't want. Below is the code. Thanks for your help.

let array=[];


function listToArray(list) {
  
  while(list.value) {
    
    array.push(list.value);
    delete list.value;
    list=list.rest;
    listToArray(list);
    
   }
  
  return array;
 
  
}

console.log(listToArray({value: 10, rest: {value: 20, rest: {
value:  30
rest:   {value: 50, rest: {value: 70, …}}));

Answer 1

You could take a recursive approach without using an array for collecting nested items.

 function nestedObjToArray({ value, rest }) { return [value, ...(rest ? nestedObjToArray(rest) : [])]; } console.log(nestedObjToArray({ value: 10, rest: { value: 20, rest: { value: 30, rest: { value: 50, rest: { value: 70 } } } } }));

Answer 2

I'd use a default 2nd parameter that defaults to the empty array, then pass it along:

 function nestedObjToArray(obj, arr = []) { arr.push(obj.value); if (obj.rest) { nestedObjToArray(obj.rest, arr); } return arr; } console.log(nestedObjToArray({ value: 10, rest: { value: 20, rest: { value: 30, rest: { value: 50, rest: { value: 70, } } } } }))

This way, the array gets created the first time the function is called, because the function isn't initially called with a second parameter. On subsequent recursive calls, the created array is passed down as the second parameter.

Note that "list" is not something with a generally-understood universal meaning in JS. Some might think you're referring to a plain array. Since what you're passing is a nested object, you might call the function nestedObjToArray or something similar.

If you can't pass another parameter, you can create the array on the final recursive call and return it:

 function nestedObjToArray(obj) { const arr = obj.rest ? nestedObjToArray(obj.rest) : []; arr.unshift(obj.value); return arr; }; console.log(nestedObjToArray({ value: 10, rest: { value: 20, rest: { value: 30, rest: { value: 50, rest: { value: 70, } } } } }))