Checking for an item in an array

I'm having trouble finding the right way to find if an item from a for loop is in an array. Let's say I have a for loop that is iterating through some results. If they are in an array:

 ctids = {};

continue to the next step in the for loop, but if not, push them to the array and do something else. What is the correct syntax for this?

for (var i=0;i<results.features.length;i++){
          ACS_BG = results.features[i].attributes.BLKGRPCE;
          ACS_ST = results.features[i].attributes.STATEFP;
          ACS_CNTY = results.features[i].attributes.COUNTYFP;
          ACS_TRCT = results.features[i].attributes.TRACTCE;

    if ACS_TRCT exists in ctids { //This is where I am having trouble. 
        continue;       //skip the rest of the if statement
    } else {
        ctids.push(ACS_TRCT);
    //   do something else;
    };
};