使用if / else vs.switch

Question

I'm wondering if it would be better to use a switch statement in my specific case.

I'm writing an Alexa Custom Skill, and I need to "redirect" to the appropriate intent depending on the available information (aka slots). Below is what I have currently (using if/else ):

if (_type === "IntentRequest") {

    this.handler.state = states.START;
    if (_slots.indexOf("address") != -1) {
        this.emitWithState("GoingToAddressIntent");
    } else if (_slots.indexOf("place") != -1) {
        this.emitWithState("GoingToPlaceIntent");
    } else if (_slots.indexOf("type") != -1) {
        this.emitWithState("GoingToTypeIntent");
    } else if (_slots.indexOf("purpose") != -1) {
        this.emitWithState("GoingToPurposeIntent");
    } else {
        this.emit("Unhandled");
    }

}

I expect _slots to be an array of any permutations of the four elements, [ "address", "place", "type", "purpose" ] . Therefore, it could be anything from [ "address" ] to [ "place", "purpose" ] to etc. etc., but always in the same order (eg [ "purpose", "address" ] would never happen).

The order of the comparisons matters because there is a "hierarchy" of information; so if the "address" slot is present, I have to emit the "GoingToAddressIntent" regardless of what other slots are available. Given this requirement, I thought using a switch statement maybe more straightforward and readable despite having to have a few extra lines of code to "convert" the array of strings to an array of booleans. It clearly lays out the hierarchy & make sure they are evaluated in order. I could do:

if (_type === "IntentRequest") {

    this.handler.state = states.START;
    slots = [ 
        _slots.indexOf("address") != -1,
        _slots.indexOf("place") != -1,
        _slots.indexOf("type") != -1,
        _slots.indexOf("purpose") != -1
    ]

    switch(slots.indexOf(true)) {
    case 0:
        this.emitWithState("GoingToAddressIntent");
        break;
    case 1:
        this.emitWithState("GoingToAddressIntent");
        break;
    case 2:
        this.emitWithState("GoingToTypeIntent");
        break;
    case 3:
        this.emitWithState("GoingToPurposeIntent");
        break;
    default:
        this.emit("Unhandled");
    }

}

... in which case I have an extra line to define the array of booleans, use indexOf() to get the index of the first occurrence of a true literal (because all 4 slots are always in the order of hierarchy), and run it through the switch statement. However I wanted ask experts on their ideas of what best programming practice is in this case and the reasoning behind it because I want this to become a long-term project that is maintainable, and also I believe I can learn something from their insights.

---

Please leave a comment if you think this should be migrated to another community on SE, but from my research (although 3 years old) I believe this should be fine (I'm just not 100% confident on this).

Answer 1

If they're always in the order of precedence in _slots , maybe you could make a hash map to the state you're going to emit...

const map = {
  address: "GoingToAddressIntent",
  place: "GoingToPlaceIntent",
  type: "GoingToTypeIntent",
  purpose: "GoingToPurposeIntent"
};

const state = map[_slots[0]];
if (state) {
  this.emitWithState(state);
} else {
  this.emit("Unhandled");
}

Answer 2

I wouldn't go with your example of the switch statement. People could understand what you're attempting to do, but it does seem pretty convoluted. I use switch statements pretty liberally, mostly in backend code, and I think it could work fine here. A group of if/else is fine too, since there's only 4 cases you need to work through. Lets roll with the switch statement since that's what you're asking about.

Based on your explanation, the order is always going to be the same, although the first value you get may be different. So the solution would be to simply grab the first value, and switch over that.

if (!!slots.length) {
  var keyword = slots[0];
  switch (keyword) {
    case 'address':
        this.emitWithState("GoingToAddressIntent");
        break;
    case 'place':
        this.emitWithState("GoingToPlaceIntent");
        break;
    case 'type':
        this.emitWithState("GoingToTypeIntent");
        break;
    case 'purpose':
        this.emitWithState("GoingToPurposeIntent");
        break;
    default:
        this.emit('Unhandled'); // I typically throw an exception here
  }
}