将值存储在多维数组中（JavaScript）

Question

What is the quickest way to store a value in an array at position, let's say, [30][1][0] ...

I tried var myArray[30][1][0] = new Date(); but it won't work without doing var myArray = []; myArray[30] = []; myArray[30][1] = []; var myArray = []; myArray[30] = []; myArray[30][1] = []; beforehand.

What am I doing wrong?

EDIT: My issue is that I don't know, first, if the variable myArray already exists and, second, if there's already a value at position, let's say, [29][2][15], which I wouldn't want to overwrite.

Answer 1

The correct way to do what you want would be:

var array = [[[new Date()]]];

However, the resulting multidimensional array will not have exactly 30 elements on the third dimension and 1 on the second, like you had in your original code. Not sure if that's going to be a problem for you.

When you do myArray[30][1][0] , JavaScript is trying to access element 30 in the array myArray , but, since myArray variable was not initialized to anything yet, it has the value undefined . So your code is equivalent with:

var myArray;
myArray[30] = new Date();

This code is going to issue the error Cannot set property '30' of undefined .

Edit: If you want to make sure the array exists before assigning values and avoid overriding existing values, the most elegant way I can think of is:

// Store a value in the array, at index (posX, posY, posZ), only if
// that position in the array is currently empty
function storeDateNoOverride(theArray, posX, posY, posZ, date) {
    var theArray = theArray || [];
    theArray[posX] = theArray[posX] || [];
    theArray[posX][posY] = theArray[posX][posY] || [];
    theArray[posX][posY][posZ] = theArray[posX][posY][posZ] || date;
}

// If variable theArray doesn't have a value yet, assign it to a new array
var theArray = theArray || [];

storeDateNoOverride(theArray, 30, 1, 0, new Date());

Answer 2

What is the quickest way to store a value in an array at position, let's say, [30][1][0] ...

The classic way is

array[30] = array[30] || [];
array[30][1] = array[30][1] || [];
array[30][1][0] = array[30][1][0] || [];
array[30][1][0].push(value);

or the equivalent in suitably parameterized fashion.

You could make a little helper:

function subarray(array, n) {
  return array[n] = array[n] || [];
}

Then you could write

subarray(subarray(subarray(array, 30), 1), 0).push(value)

You could also write the helper to do all the accessing and checking in one fell swoop:

function getNestedElement(array, firstIndex, ...indices) {
  if (firstIndex === undefined) return array;
  return getNestedElement(array[firstIndex] = array[firstIndex] || [], ...indices);
}

And then write

getNestedElement(array, 30, 1, 0).push(value)

Answer 3

You can not initialize a nested object like that. There are two ways. If you know your ND array size in advance you should first instantiate it as follows;

 Array.prototype.clone = function(){ return this.map(e => Array.isArray(e) ? e.clone() : e); }; function arrayND(...n){ return n.reduceRight((p,c) => c = (new Array(c)).fill().map(e => Array.isArray(p) ? p.clone() : p )); } var arr = arrayND(5,5,5,void 0); // create 5x5x5 array and fill each cell with "undefined" arr[3][2][1] = arr[3][2][1] || new Date(); // if falsey value then insert current date. console.log(arr); 

However if you don't know the multidimensional size in advance and would like to grow your array dynamically you can use a function like getNestedValue() and setNestedValue() . In this particular case i will extend the Object.prototype . In production code this should be done through Object.defineProperty() tool though.

 Object.prototype.getNestedValue = function(...a) { return a.length > 1 ? (this[a[0]] !== void 0 && this[a[0]].getNestedValue(...a.slice(1))) : this[a[0]]; }; Object.prototype.setNestedValue = function(...a) { a.length > 2 ? typeof this[a[0]] === "object" && this[a[0]] !== null ? this[a[0]].setNestedValue(...a.slice(1)) : (this[a[0]] = typeof a[1] === "string" ? {} : new Array(a[1]), this[a[0]].setNestedValue(...a.slice(1))) : this[a[0]] = a[1]; return this; }; var arr = []; arr.getNestedValue(30,1,0) || arr.setNestedValue(30,1,0,new Date()); // if exists don't overwrite it; console.log(arr[30][1][0]);