How to get the length of an object after checking it is an array in Javascript

Question

I have an array of objects, there is one field in each object could be boolean or a string of array, for instance

myObjects=[{id: "fruit", selected: true}, {id: "veggie", selected: ["tomato", "cucumber", "potato"]}, {id: "diary", selected:[]}]

I would like to filter out the objects that have selected either not empty or true. The result would be:

result=[{id: "fruit", selected: true}, {id: "veggie", selected: ["tomato", "cucumber", "potato"]}]

Here is my code:

for (const object in myObjects) {
if (!object[selected] || (Array.isArray(object[selected] && object[selected].length ===0)) continue
...
}

But objects with selected is empty array won't get filter out. I got a typescript error

Property 'length' does not exist on type 'boolean | string[]'.
  Property 'length' does not exist on type 'false'.

Can anyone help?

You use a for...in loop on an array of objects? Should this not be for...of loop? — Sean
– Sean, Commented Aug 12, 2022 at 18:01
You could use a .filter() function. Something like myObjects.filter(obj => (typeof obj.selected === 'boolean' && obj.selected) || obj.selected.length > 0) to filter out any boolean false or empty arrays — ElderTale
– ElderTale, Commented Aug 12, 2022 at 18:03
There is a bad condition grouping, must be if (!object[selected] || Array.isArray(object[selected]) && object[selected].length === 0) continue — n--
– n--, Commented Aug 12, 2022 at 18:06
From what I tested, this should be a SyntaxError (in the if there's 3 opening parentheses and only 2 closing). So maybe there's something missing from the code posted here, compared to what you actually ran. It could be because instead of "continue;", you used a function or something else that is acceptable inside if (edit: I think it would only work with an operator actually). If you could clarify that, it'll probably be helpful. But yeah, what @n-- said matches with the behaviour you presented, it looks like the length part is inside the isArray function. — user19513069
– user19513069, Commented Aug 12, 2022 at 18:20
If you're going to use bracket notation, you need to use object["selected"] otherwise it's looking for a variable called selected. — mykaf
– mykaf, Commented Aug 12, 2022 at 20:32

Sean · Accepted Answer · 2022-08-12 18:05:12Z

3

You can use the filter method on the myObjects array

const res = myObjects.filter((object) => object.selected.length > 0 || object.selected === true )

answered Aug 12, 2022 at 18:05

Sean

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2 Comments

Tom Over a year ago

The specific error he's seeing is because of typescript, so something like: myObjects.filter(obj => Array.isArray(obj.selected) ? obj.selected.length > 0 : obj.selected === true) is required

Tom Over a year ago

Agreed, was just pointing out that this answer doesn't solve the issue Property 'length' does not exist on type 'boolean | string[]'.

TAHER El Mehdi · Accepted Answer · 2022-08-12 18:16:14Z

You could do it with loop for...of

add object to result array only if selected property is boolean and true:

(typeof object.selected === "boolean" && object.selected)

or selected property is an array and have elements:

(Array.isArray(object["selected"]) && object["selected"].length !== 0)

const myObjects = [{
    id: "fruit",
    selected: true,
  },
  {
    id: "veggie",
    selected: ["tomato", "cucumber", "potato"],
  },
  {
    id: "diary",
    selected: [],
  },
];
let result = [];
//                vv replace 'in' with 'of' since you loop over an array not an object
for (const object of myObjects) {
  if (
    (typeof object.selected === "boolean" && object.selected) ||
    (Array.isArray(object["selected"]) && object["selected"].length !== 0)
  )
    result.push(object);
}
console.log(result);

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