Check if an array has duplicates

To check if an array has duplicates, you can combine the some() function and an empty object:

javascript
let hasDuplicates = ['a', 'b', 'a'].some(e => {
    
    if(o[e]){
        
        // If the element exists in the object, return true
        // The some function will break after this line
        return true;
        
    }
    else{
        
        // If the element doesn't exist, create it
        o[e] = true;
     
        // The some function will continue after this line
        // on returning false
        return false;
        
    }
    
}, o = {});

In one line of code:

javascript
let hasDuplicates = ['a', 'b', 'a'].some(e => o[e] ? true : (o[e] = true, false), o = {}); // true