A JS function to enumerate all arrangements of some letters

arrangements.js

function arrangements(letters){
    if (letters.length == 1){
        return [letters];
    } 
    
    else if (letters.length == 2){
        return [letters[0] + letters[1], letters[1] + letters[0]];
    } 
    
    else {
        var out = [];
        
        for (var i=0; i<letters.length; i++){
            var letter = letters[i];
            
            arrangements(letters.slice(0, i) + letters.slice(i+1, letters.length)).forEach(function(arr){
                out.push(letter + arr);
            });
        }
        
        return out;
    }
}