[javascript] Check variable equality against a list of values

I'm checking a variable, say foo, for equality to a number of values. For example,

if( foo == 1 || foo == 3 || foo == 12 ) {
    // ...
}

The point is that it is rather much code for such a trivial task. I came up with the following:

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

but also this does not completely appeal to me, because I have to give redundant values to the items in the object.

Does anyone know a decent way of doing an equality check against multiple values?

If you have access to Underscore, you can use the following:

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

contains used to work in Lodash as well (prior to V4), now you have to use includes

if (_.includes([1, 3, 12], foo)) {
  handleYellowFruit();
}

I simply used jQuery inArray function and an array of values to accomplish this:

myArr = ["A", "B", "C", "C-"];

var test = $.inArray("C", myArr)  
// returns index of 2 returns -1 if not in array

if(test >= 0) // true

For posterity you might want to use regular expressions as an alternative. Pretty good browser support as well (ref. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/match#Browser_compatibility)

Try this

if (foo.toString().match(/^(1|3|12)$/)) {
    document.write('Regex me IN<br>');
} else {
    document.write('Regex me OUT<br>');
}

This is a little helper arrow function:

const letters = ['A', 'B', 'C', 'D'];

function checkInList(arr, val) {
  return arr.some(arrVal => val === arrVal);
}

checkInList(letters, 'E');   // false
checkInList(letters, 'A');   // true

More info here...

You could use an array and indexOf:

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

Also, since the values against which you're checking the result are all unique you can use the Set.prototype.has() as well.

var valid = [1, 3, 12];_x000D_
var goodFoo = 3;_x000D_
var badFoo = 55;_x000D_
_x000D_
// Test_x000D_
console.log( new Set(valid).has(goodFoo) );_x000D_
console.log( new Set(valid).has(badFoo) );

var a = [1,2,3];

if ( a.indexOf( 1 ) !== -1 ) { }

Note that indexOf is not in the core ECMAScript. You'll need to have a snippet for IE and possibly other browsers that dont support Array.prototype.indexOf.

if (!Array.prototype.indexOf)
{
  Array.prototype.indexOf = function(searchElement /*, fromIndex */)
  {
    "use strict";

    if (this === void 0 || 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)
        n = 0;
      else if (n !== 0 && n !== (1 / 0) && n !== -(1 / 0))
        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;
  };
}

Now you may have a better solution to resolve this scenario, but other way which i preferred.

const arr = [1,3,12]
if( arr.includes(foo)) { // it will return true if you `foo` is one of array values else false
  // code here    
}

I preferred above solution over the indexOf check where you need to check index as well.

includes docs

if ( arr.indexOf( foo ) !== -1 ) { }

I liked the accepted answer, but thought it would be neat to enable it to take arrays as well, so I expanded it to this:

Object.prototype.isin = function() {
    for(var i = arguments.length; i--;) {
        var a = arguments[i];
        if(a.constructor === Array) {
            for(var j = a.length; j--;)
                if(a[j] == this) return true;
        }
        else if(a == this) return true;
    }
    return false;
}

var lucky = 7,
    more = [7, 11, 42];
lucky.isin(2, 3, 5, 8, more) //true

You can remove type coercion by changing == to ===.

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)) {
    // ...
}

This is easily extendable and readable:

switch (foo) {
    case 1:
    case 3:
    case 12:
        // ...
        break

    case 4:
    case 5:
    case 6:
        // something else
        break
}

But not necessarily easier :)

You can write if(foo in L(10,20,30)) if you define L to be

var L = function()
{
    var obj = {};
    for(var i=0; i<arguments.length; i++)
        obj[arguments[i]] = null;

    return obj;
};