Javascript: The prettiest way to compare one value against multiple values

Don't try to be too sneaky, especially when it needlessly affects performance. If you really have a whole heap of comparisons to do, just format it nicely.

if (foobar === foo ||
    foobar === bar ||
    foobar === baz ||
    foobar === pew) {
     //do something
}

What i use to do, is put those multiple values in an array like

var options = [foo, bar];

and then, use indexOf()

if(options.indexOf(foobar) > -1){
   //do something
}

for prettiness:

if([foo, bar].indexOf(foobar) +1){
   //you can't get any more pretty than this :)
}

and for the older browsers:
( https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/IndexOf )

if (!Array.prototype.indexOf) {
    Array.prototype.indexOf = function (searchElement /*, fromIndex */ ) {
        "use strict";
        if (this == null) {
            throw new TypeError();
        }
        var t = Object(this);
        var len = t.length >>> 0;
        if (len === 0) {
            return -1;
        }
        var n = 0;
        if (arguments.length > 0) {
            n = Number(arguments[1]);
            if (n != n) { // shortcut for verifying if it's NaN
                n = 0;
            } else if (n != 0 && n != Infinity && n != -Infinity) {
                n = (n > 0 || -1) * Math.floor(Math.abs(n));
            }
        }
        if (n >= len) {
            return -1;
        }
        var k = n >= 0 ? n : Math.max(len - Math.abs(n), 0);
        for (; k < len; k++) {
            if (k in t && t[k] === searchElement) {
                return k;
            }
        }
        return -1;
    }
}

Since nobody has added the obvious solution yet which works fine for two comparisons, I'll offer it:

if (foobar === foo || foobar === bar) {
     //do something
}

And, if you have lots of values (perhaps hundreds or thousands), then I'd suggest making a Set as this makes very clean and simple comparison code and it's fast at runtime:

// pre-construct the Set
var tSet = new Set(["foo", "bar", "test1", "test2", "test3", ...]);

// test the Set at runtime
if (tSet.has(foobar)) {
    // do something
}

For pre-ES6, you can get a Set polyfill of which there are many. One is described in this other answer.