Differentiating between undefined array elements and empty slots can be tricky - but fear not, we

Question

Differentiating between undefined array elements and empty slots can be tricky - but fear not, we

Imagine being a user script developer without control over the on-page JavaScript. The page is generating arrays with random lengths and filling them with various values, including some falsy ones like undefined. It's possible for some elements to not have assigned values, leading to empty slots within the array.

Here is a simple example (for Firefox console):

var arr = new Array(3);
arr[0] = null;
arr[1] = undefined;
console.log(arr);               \\ Array [ null, undefined, <1 empty slot> ]
console.log(arr[1]);            \\ undefined
console.log(arr[2]);            \\ undefined
console.log(arr[1] === arr[2]); \\ true
console.log(typeof arr[1]);     \\ undefined
console.log(typeof arr[2]);     \\ undefined

In this scenario, Firefox displays undefined and empty slots differently, although JavaScript treats them as identical.

If your goal is to clean up such an array by removing all empty slots while preserving any undefined elements, how would you accomplish that?

javascript arrays undefined

Answer 1

Answer №1

To determine if an array contains a specific key, you can utilize the in operator. This operator will evaluate to false for empty slots within the array, but true for slots that have values, even if the value is undefined:

var arr = new Array(3);
arr[0] = null;
arr[1] = undefined;

1 in arr; // true
2 in arr; // false

It's important to note that using this method does not allow you to differentiate between empty slots and slots located beyond the actual end of the array. However, if you already know the length of the array, it becomes unnecessary to test 3 in arr, since you are aware that index 3 is not part of the array.

If you want to filter out the empty slots present in the array, you can achieve this with the following code snippet:

arr = arr.filter((_, i) => i in arr);

Answer 2

To determine if an array contains a specific key, you can utilize the in operator. This operator will evaluate to false for empty slots within the array, but true for slots that have values, even if the value is undefined:

var arr = new Array(3);
arr[0] = null;
arr[1] = undefined;

1 in arr; // true
2 in arr; // false

It's important to note that using this method does not allow you to differentiate between empty slots and slots located beyond the actual end of the array. However, if you already know the length of the array, it becomes unnecessary to test 3 in arr, since you are aware that index 3 is not part of the array.

If you want to filter out the empty slots present in the array, you can achieve this with the following code snippet:

arr = arr.filter((_, i) => i in arr);

Answer 3

Answer №2

You have the option to utilize Array#forEach, which excludes sparse elements.

var arr = new Array(3);
arr[0] = null;
arr[1] = undefined;
console.log(arr); 

var withoutSparse = [];

arr.forEach(function (a, i) {
    console.log(i);
    withoutSparse.push(a);
});
console.log(withoutSparse);

.as-console-wrapper { max-height: 100% !important; top: 0; }

Answer 4

You have the option to utilize Array#forEach, which excludes sparse elements.

var arr = new Array(3);
arr[0] = null;
arr[1] = undefined;
console.log(arr); 

var withoutSparse = [];

arr.forEach(function (a, i) {
    console.log(i);
    withoutSparse.push(a);
});
console.log(withoutSparse);

.as-console-wrapper { max-height: 100% !important; top: 0; }

Differentiating between undefined array elements and empty slots can be tricky - but fear not, we

Answer №1

Answer №2

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