Get first property of an object that matches the condition

Question

So I have a structure like that:

Foo: {
    A: Array[0],
    B: Array[0],
    C: Array[1]
}

where [X] is length of the array, but Foo is an object, not an Array, therefore I can't use Array method on it.

How do I get first element (letter in this case) which has length > 0 ?

for (let letter in Foo) {
    if (letter.length > 0) {
        let match = letter;
    }
}

I tried something like this (this is simplified version), but it just returns all properties of Foo .

Answer 1

I'm glad you're using ES6. In this case you can use Object.keys to get an array of all the object's keys and Array.prototype.find to find the first element with a specific property:

var obj = {
  a: [],
  b: [],
  c: [
    2,
    3
  ],
  d: [],
  e: [
    1
  ]
};

Object.keys(obj).find(a => obj[a].length > 0); // The letter "c" which contains the first non-empty array.

obj[Object.keys(obj).find(a => obj[a].length > 0)]; // Array [2, 3] itself

Note that there's no consistent “first” element in an object across implementations.