JavaScript switch case using enum

Question

I have an "enum" declared like so:

var PlaceType = {
        PASSABLE_TERRAIN: 1,
        IMPASSABLE_TERRAIN: 0,
        SOMEWHAT_PASSABLE_TERRAIN: 2,
        PATH: 3
    };

and a function declared like this:

setPlaceType(placeType) {
        this.clear = false;
        this.placeType = placeType;

        alert("before switch "+(PlaceType.SOMEWHAT_PASSABLE_TERRAIN==this.placeType));
        switch(this.placeType) {
        case PlaceType.PASSABLE_TERRAIN: {
            alert("Case PASSABLE");
            break;
        }
        case PlaceType.IMPASSABLE_TERRAIN: {
            alert("Case IMPASSABLE");
            break;
        }
        case PlaceType.SOMEWHAT_PASSABLE_TERRAIN: {
            alert("Case SOMEWHAT_PASSABLE");
            break;
        }
        case PlaceType.PATH: {
            alert("Case PATH");
            break;
        }
        default: {
            alert("case default");
        }
        }
    }

if I call it like this:

setPlaceType(1);

I get the following alerts: "before switch true", "case default"

if I call it like this:

setPlaceType(2);

I get the following alerts: "before switch false", "case default"

In other words, the function is called with the proper argument, which, when doing (what it seems to me to be) the same comparison as the switch but via "==" I get correct behavior, but the switch never matches the values to the appropriate case. Does anybody know why?

Solution 1

The comparison operator will cast both operands to strings if either operator is a string. If you pass in a string, you are comparing string == number which will cast the number to a string and, in the case of passing the string '2', it will be true.

switch case comparison uses the identity operator === and will fail if the operands are not the same type.

long story short, make sure you are always passing a number if your cases are comparing against numbers, you can double check like this:

setPlaceType(placeType) {
    if (typeof placeType !== 'number') {
        throw new Error('You must pass a number to setPlaceType!');
    }
    ...
}

also, you should be calling your function like this:

setPlaceType(PlaceType.PASSABLE_TERRAIN);

otherwise there's not really any point to using the "enumeration" (i use that term loosely) object.

Solution 2

The matching case is determined using the === identity operator, not the == equality operator. The expressions must match without any type conversion. It would fail if you are passing in a string and not an integer.

setPlaceType(1);  //"Case PASSABLE"
setPlaceType("1");  //"case default"

Example running your code with the above lines: jsFiddle

So if you are saying it is failing, you are probably comparing a string to a number. Use parseInt.

this.placeType = parseint(placeType,10);

Solution 3

Another way to use enum for switch case:

   const PlaceType = Object.freeze({
            PASSABLE_TERRAIN: 1,
            IMPASSABLE_TERRAIN: 0,
            SOMEWHAT_PASSABLE_TERRAIN: 2,
            PATH: 3
        });

   function setPlaceType(placeType){
      return({
              0:"Case IMPASSABLE_TERRAIN",
              1:"Case PASSABLE_TERRAIN",
              2:"Case SOMEWHAT_PASSABLE_TERRAIN",
              3:"PATH"      
              }[placeType]);
     }

Solution 4

When you are doing the comparison using == js is using type-coercion to cast the 2 operands to an intermediate type, string in this case, and thus compare them successfully.

So to get the effect to work with your switch statement, you will need to cast as such

this.placeType = parseint(placeType);

What you also got to learn here is that it is not really an ideal practice to compare 2 values in javascript using the == operator, instead use the === operator which also checks for the types to be the same. So in your case,

alert("before switch "+(PlaceType.SOMEWHAT_PASSABLE_TERRAIN==this.placeType));

would have failed if you would have used === as you are comparing an int and string

Working demo here: http://jsfiddle.net/pratik136/ATx8c/

View more solutions

Comments

[Shivan Dragon]

Thank you, that was it. However the fact that the "==" check passes but the switch check does not is really counter intuitive. I understand now that the switch actually makes a "===" check, but still.

[Shivan Dragon]

well, yes, 'cause it also explains about the "==" and "===", and I think it will be more helpful to some other mega noob like me if/when he/she stumbles over this question.

[Samuel]

I can be wrong but I think that in javascript, the type coercion always try to cast both types in number except if they are the same type; not in string. Please read this good article: javascriptweblog.wordpress.com/2011/02/07/…

[rusty]

How do I use above scenario to handle undefined or value equal or greater than 4