Difficulty in understanding problem solving

Question

My problem is this; I was solving some exercises of the hacker rank platform and I came across a question that asked me to identify the only value in an array that did not repeat. I couldn’t think of any logic to solve the problem and decided to research, I found some resolutions for the exercise but I didn’t understand the logic behind the codes, could help me understand them ?

Because this first code has a supporting text I was able to partially understand. I understood that the method .filter creates a new array if you pass the test performed by the higher-order function, so this higher-order function would be the function(value)?, also did not understand why it is inside the parentheses, it is the parameter of the method .filter?. I looked at the documentation on these methods .indexOf and the .lastIndexOf but I couldn’t understand exactly how they work within that code.

function lonelyInteger(a) {
  let unique = a.filter(function(value){
    return a.indexOf(value) === a.lastIndexOf(value)
  })
  
  return unique[0];
}

But this second code is very abstract for me, I don’t understand why {} in the variable counts, why result is loose like that under the let counts = {} and the logic of ties for didn’t make sense to me.

function lonelyinteger(a) {
    // Write your code here
    
    let counts = {},
        result;
    
    for(let i in a) {
        
        let num = a[i];
        counts[num] = counts[num] ? counts[num] + 1 : 1;
        
        for(let j in counts){
        
            if(counts.hasOwnProperty(num)) {
                
                if(counts[j] === 1){
                    result = j;    
                }    
            }
        }
    }
       
    return result;
}

I know my doubts are basic, but I don’t want to resort to the easiest way to just do Ctrl C + Ctrl V, I really want to understand how to solve the issue.

Source:

first code: https://levelup.gitconnected.com/javascript-algorithm-lonely-integer-4397cd8b6ffc

2nd code: discussion tab of the exercise itself on the site Hacker Rank