javascript reduce(),keys(), filter() method

April 2, 2024

While solving a problem to find missing numbers from an array.

let arr = [15,3,5,6,7,2,1,8,12];

let findMissing = (arr)=>{
    let res = arr.reduce((acc,n)=>{
        acc[n]=1;
        return acc;
    },[])
    
    return [...res.keys()].filter((i)=>{
        if(res[i]!==1) return i
    })
}

console.log(findMissing(arr));

output:[4, 9, 10, 11, 13, 14]

Issue is why output does not have zero in it: [0,4, 9, 10, 11, 13, 14]

>Solution :

because you return i in the filter method (you return 0).
i === 0 && 0 == false it is filtered out because when returning a falsy value for a filter function it will strip it.

if you return true like in my example it works.

let arr = [15,3,5,6,7,2,1,8,12];

let findMissing = (arr)=>{
    let res = arr.reduce((acc,n)=>{
        acc[n]=1;
        return acc;
    },[])
    
    return [...res.keys()].filter((i)=>{
        return res[i] !== 1
    })
}

console.log(findMissing(arr));