As a naive approach something like this:

    // Throw an error if inputKey is not correct.
    // inputKey and correctKey are both strings
    function checkApiKey(inputKey, correctKey) {
      var isMatch = true;
      for (var i = 0; i < inputKey.length; i++)
        if (inputKey.charAt(i) !== correctKey.charAt(i)) {
          isMatch = false;
        }
      }
      if (!isMatch) {
        throw new Error("wrong key");
      }
    }

So what I meant is, don't return early when isMatch = false, instead walk the entire key regardless.