I want to filter the object of booleans and then select the key name of the true item in the most efficient way, drawing a blank on selecting the active key name.

const winners = {
lg: false
md: true
sm: false
xs: false
}

const active = Object.entries(winners).filter(([key, bool]) => !!bool)[0][0]
//this seems quite ugly, can I chain active or flatten the list to get the key 'md'?

Maybe something like this:

const [key, value] = Object.entries(winners).filter(([key, bool]) => !!bool)[0]

I want to filter the object of booleans and then select the key name of the true item in the most efficient way, drawing a blank on selecting the active key name.

const winners = {
lg: false
md: true
sm: false
xs: false
}

const active = Object.entries(winners).filter(([key, bool]) => !!bool)[0][0]
//this seems quite ugly, can I chain active or flatten the list to get the key 'md'?

Maybe something like this:

const [key, value] = Object.entries(winners).filter(([key, bool]) => !!bool)[0]

• javascript

• Was messing around with the oute with some trial and error @trincot – styler Commented Aug 19, 2020 at 19:23

5 Answers 5

You can use a for loop:

let winnerKey = null;

for(let key in winners) {
    if(winners[key]) {
       winnerKey = key;
       break;
    }
}

If you want a readable solution which gets all true keys, then

true_keys = Object.keys(winners).filter(key=>winners[key]);

isn't a bad choice.

If you want speed them @domenikk's answer is the way to go, but otherwise readability is usually preferable over maximum efficiency (I mean, you are using JS).

Can use pickBy, with some sort of destructuring:

function pickBy(object) {
    const obj = {};
    for (const key in object) {
        if (object[key]) {
            obj[key] = object[key];
        }
    }
    return obj;
}
const [key, value] = Object.entries(pickBy(winners))[0]

• https://github./you-dont-need/You-Dont-Need-Lodash-Underscore#_pickby

You could also use a reduce:

Object.entries(winners).reduce((p, [key, value]) => value ? key : p, null)

"md"

Object.entries({}).reduce((p, [key, value]) => value ? key : p, null)

null

I would use something like this:

const winners = {
lg: false,
md: true,
sm: false,
xs: false,
}
const active = Object.keys(winners).filter(key => winners[key])[0]

Do not you think that array of object is more suitable store for winners? Like

const winners = [
  {
    name: "lg",
    is_winner: false
  },
  {
    name: "md",
    is_winner: true
  },
  ...
]

That way you could easily manage the list of winners - add, delete, filter etc. To create that kind of storage you can use

const winnersArray = Object.keys(winners).map(key => ({ "name": key, "is_winner": winners[key] }))

You can stop iterating as soon as a true is found.

const test = {
  lg: false,
  md: true,
  sm: false,
  xs: false
}

const winner = Object.keys(test)[Object.values(test).indexOf(true)]

console.log(winner)