Check variable equality against a list of values

You could use an array and indexOf:

if ([1,3,12].indexOf(foo) > -1)

In ECMA2016 you can use the includes method. It's the cleanest way I've seen. (Supported by all major browsers)

if([1,3,12].includes(foo)) {
    // ...
}

Using the answers provided, I ended up with the following:

Object.prototype.in = function() {
    for(var i=0; i<arguments.length; i++)
       if(arguments[i] == this) return true;
    return false;
}

It can be called like:

if(foo.in(1, 3, 12)) {
    // ...
}

---

Edit: I came across this 'trick' lately which is useful if the values are strings and do not contain special characters. For special characters is becomes ugly due to escaping and is also more error-prone due to that.

/foo|bar|something/.test(str);

To be more precise, this will check the exact string, but then again is more complicated for a simple equality test:

/^(foo|bar|something)$/.test(str);