JavaScript -how to get array value with some condition?

I'm not sure why you're stuck at this stage since it is quite trivial.

In this line:

const getin = (day) =day.map (d=> ds.indexOf(d)>=0 && data.days[d].checkin >= data.days[d].checkout);

you are essentially allotting the result of ds.indexOf(d)>=0 && data.days[d].checkin >= data.days[d].checkout to getin and day.

ds.indexOf(d)>=0 && data.days[d].checkin >= data.days[d].checkout is a boolean since it is the output of two checks.

You could use something like this to use the check instead of returning the check itself: ds.indexOf(d)>=0 && data.days[d].checkin >= data.days[d].checkout ? d : false. ? is a condition operator which helps reduce typing if else statements.

The modified code:

 let data = 
{
    "days": {
        "Monday": {
            "checkin": "23:00",
            "checkout": "10:00"
        },
        "Tuesday": {
            "checkin": "07:00",
            "checkout": "14:00"
        },
        "Wednesdy": {
            "checkin": "07:00",
            "checkout": "04:00"
        },
    }
}
let day = ['Monday','Tuesday','Wednesdy'];
const ds = Object.keys(data.days);

const getin = (day) =day.map (d=> ds.indexOf(d)>=0 && data.days[d].checkin >= data.days[d].checkout ? d : false); 


console.log("===Results===",getin);

You can do the following by pushing the object in a new array when the filter condition is true,

 let data = 
{
    "days": {
        "Monday": {
            "checkin": "23:00",
            "checkout": "10:00"
        },
        "Tuesday": {
            "checkin": "07:00",
            "checkout": "14:00"
        },
        "Wednesdy": {
            "checkin": "07:00",
            "checkout": "04:00"
        },
    }
}
let day = ['Monday','Tuesday','Wednesdy'];
const ds = Object.keys(data.days);
const daysRes = [];
const getin = (day) =day.map (d=> {
  if(ds.indexOf(d)>=0 && data.days[d].checkin >= data.days[d].checkout) {
    daysRes.push(data.days[d]);
    return true;
  }
  return false;
}); 


console.log("===Results===",getin);
console.log("===ResultsDays===",daysRes);

Use array.filter instead of array.map.

let data = {
  days: {
    Monday: {
      checkin: "23:00",
      checkout: "10:00"
    },
    Tuesday: {
      checkin: "07:00",
      checkout: "14:00"
    },
    Wednesdy: {
      checkin: "07:00",
      checkout: "04:00"
    }
  }
};
let day = ["Monday", "Tuesday", "Wednesdy"];
const ds = Object.keys(data.days);

const getin = Object.values(data.days).filter(d => (d.checkin >= d.checkout));

console.log("===Results===", getin);

Why your code returns ["true","false","true"]?

You are making use of array.map you are returning the result of the operation ds.indexOf(d)>=0 && data.days[d].checkin >= data.days[d].checkout This response is always boolean, either true or false.

array.map always returns a value corresponding to each node in the array. In your case it returned a boolean value after the comparison.

Go for array.filter, this will create a new array from an existing one by returning the entire node of the array dependig on a condition, which is your actual requirement.