Simplify if(x === y || x === z || x === a...)

Often times, I and many other programmers have to deal with something similar to:

if (x === a || x === b || x === c || x === d) {
   // Do assignment
}

where x is an input that you are checking if it equals other variables, a, b, c, d. Sometimes the variable names are long. What's the best (or good way) to simplify this?