What's the best way to test for the existence of an array element in a structure several layers deep? For instance:
if (typeof arr[a][b][c] === 'undefined') { ...do something... }
If [a] or [b] don't exist, we won't be able to test for [c].
Are there underscore or lodash functions to handle this?
It you need it in many places you can create a function:
function get(obj, props) {
return props.reduce(function(x, p) { return x && x[p] ? x[p] : null }, obj)
}
if (get(arr, [a, b, c])) ...
Works with objects and arrays:
var obj = [0, {a: [0, 'value', 2]}, 2]
get(obj, [1, 'a', 1]) //=> 'value'
get(obj, [1, 'a', 8]) //=> null
For sparse array check, just checking an item being undefined is not sufficient. However you can use the in operator to test like
(a,b,c) in arr
var a = [];
a[10] = 1;
b = 10;
console.log(1 in a)
console.log((1,2,3) in a)
console.log(b in a)
[undefined,3,undefined] is not a sparse array.Array(3) is sparse while Array(3).fill() is not. So on V8 console for instance console.log(Array(3)) would look like [undefined × 3] while Array(3).fill() would look like [undefined, undefined, undefined] which means the first array have no keys (properties) at all while the latter has keys (0,1,2) assigned as undefined as values. A sparse array has holes in the sense of keys. In another words an array with undefined items don't qualify as a sparse array.you can do
if (arr && arr[a] && arr[a][b] && typeof arr[a][b][c] === 'undefined') { ...do something... }
or you can create a custom function
function check(arr){
let temp = arr;
for(let i=1; i<arguments.length; i++){
if(temp[arguments[i]] == undefined){
return false;
}
temp = temp[arguments[i]];
}
return true;
}
let arr= [[[1], [2]], [3]];
console.log(check(arr, 0, 0));
console.log(check(arr, 0, 0, 0, 0));For anyone finding this question after the introduction of optional chaining (circa 2020) this is now easily accomplished:
if (!arr[a]?.[b]?.[c]) {
// the element is not present. Do something.
}
Here's a demo:
let a = [];
a[5] = [[1,2,3],[4,5,6],[7,8,9]];
a[6] = [undefined, undefined, undefined];
// works for empty items
console.log(a[0]?.[2]?.[1]); // prints `undefined`, a[0] is not set
console.log(a[5]?.[2]?.[1]); // prints `8`
// works for undefined items
console.log(a[6]?.[1]?.[1]); // prints `undefined`. a[6][1] is undefined