Is There A Way To Join The Elements In An Js Array, But Let The Last Separator Be Different?

What I want is something like Array.join(separator), but which takes a second argument Array.join(separator, beforeLastElement), so when I say [foo, bar, baz].join(', ', ' or') I w

Solution 1:

There's no predefined function, because it's quite simple.

var a = ['a', 'b', 'c'];
var str = a.slice(0, -1).join(',')+' or '+a.slice(-1);

There's also a specification problem for the main use case of such a function which is natural language formatting. For example if we were to use the Oxford comma logic we would have a different result than what you're looking for:

// make a list in the Oxford comma style (eg "a, b, c, and d")
// Examples with conjunction "and":
// ["a"] -> "a"
// ["a", "b"] -> "a and b"
// ["a", "b", "c"] -> "a, b, and c"
exports.oxford = function(arr, conjunction, ifempty){
    let l = arr.length;
    if (!l) return ifempty;
    if (l<2) return arr[0];
    if (l<3) return arr.join(` ${conjunction} `);
    arr = arr.slice();
    arr[l-1] = `${conjunction} ${arr[l-1]}`;
    return arr.join(", ");
}

So it seems better to let this problem in userland.

Solution 2:

May I suggest:

['tom', 'dick', 'harry'].join(', ').replace(/, ([^,]*)$/, ' and $1')
> "tom, dick and harry"

Solution 3:

No, this is specific enough that you will have to write a custom function. The good news is, as you said, once you use Array.join to take care of all the separators, the last one will be easy enough to update.

Solution 4:

In-line solution using reduce:

[1, 2, 3, 4, 5].reduce((text, value, i, array) => text + (i < array.length - 1 ? ', ' : ' or ') + value);

=> "1, 2, 3, 4 or 5"

Solution 5:

Building off of @dystroy's answer:

function formatArray(arr){
    var outStr = "";
    if (arr.length === 1) {
        outStr = arr[0];
    } else if (arr.length === 2) {
        //joins all with "and" but no commas
        //example: "bob and sam"
        outStr = arr.join(' and ');
    } else if (arr.length > 2) {
        //joins all with commas, but last one gets ", and" (oxford comma!)
        //example: "bob, joe, and sam"
        outStr = arr.slice(0, -1).join(', ') + ', and ' + arr.slice(-1);
    }
    return outStr;
}

Example usages:

formatArray([]);                //""
formatArray(["a"]);             //"a"
formatArray(["a","b"]);         //"a and b"
formatArray(["a","b","c"]);     //"a, b, and c"
formatArray(["a","b","c","d"]); //"a, b, c, and d"