Reduce multiple ORs in IF statement in JavaScript

You could use an array of valid values and test it with indexOf:

if ([1, 3, 4, 17, 80].indexOf(x) != -1)

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Edit    Note that indexOf was just added in ECMAScript 5 and thus is not implemented in every browser. But you can use the following code to add it if missing:

if (!Array.prototype.indexOf)
{
  Array.prototype.indexOf = function(elt /*, from*/)
  {
    var len = this.length >>> 0;

    var from = Number(arguments[1]) || 0;
    from = (from < 0)
         ? Math.ceil(from)
         : Math.floor(from);
    if (from < 0)
      from += len;

    for (; from < len; from++)
    {
      if (from in this &&
          this[from] === elt)
        return from;
    }
    return -1;
  };
}

Or, if you’re already using a JavaScript framework, you can also use its implementation of that method.

switch (x) {
    case 1:
    case 3:
    case 4:
    case 17:
    case 80:
        //code
        break;
    default:
        //code
}

This is a little function I found somewhere on the web:

function oc(a) {
    var o = {};
    for (var i = 0; i < a.length; i++) {
        o[a[i]] = '';
    }
    return o;
}

Used like this:

if (x in oc(1, 3, 4, 17, 80)) {...}

I'm using it for strings myself; haven't tried with numbers, but I guess it would work.