Is there a way to join the elements in an js array, but let the last separator be different?

Question

What I want is something like Array.join(separator), but which takes a second argument Array.join(separator, beforeLastElement), so when I say [f

Answer 1

Though its a late answer, adding some approaches.

Method 1: Using Array.splice() add the last delimiter before last element and join and remove the last two ,.

function join(arr,last)
{
    if(!Array.isArray(arr)) throw "Passed value is not of array type.";
    last = last || ' and '; //set 'and' as default
    
    (arr.length>1 && arr.splice(-1,0,last));
    arr = arr.join().split("");
    arr[arr.lastIndexOf(",")]="";
    arr[arr.lastIndexOf(",")]="";
    return arr.join("");
}

console.log( join([1]) ); //single valued array
console.log( join([1,2]) ); //double valued array
console.log( join([1,2,3]) ); //more than 2 values array,
console.log( join([1,2,3],' or ') ); //with custom last delimiter
console.log( join("name") ); //Non-array type

Method 2: Using Array.reduce() to construct the string by traversing each element.

function join(arr,last)
{
    if(!Array.isArray(arr)) throw "Passed value is not of array type.";
    last=last||' and ';
    
    return arr.reduce(function(acc,value,index){
        if(arr.length<2) return arr.join();
        return acc + (index>=arr.length-2 ? index>arr.length-2 ? value : value+last : value+",");
    },"");
}

console.log( join([1]) ); //single valued array
console.log( join([1,2]) ); //double valued array
console.log( join([1,2,3]) ); //more than 2 values array,
console.log( join([1,2,3,4],' or ') ); //with custom last delimiter
console.log( join("name") ); //Non-array type