dthree dthree - 3 months ago 18
Javascript Question

Alternative to nested ternary operator in JS

I personally love ternary operators, and in my humble opinion, they make complicated expressions very easy to digest. Take this one:

word = (res.distance === 0) ? 'a'
: (res.distance === 1 && res.difference > 3) ? 'b'
: (res.distance === 2 && res.difference > 5 && String(res.key).length > 5) ? 'c'
: 'd';


However in our project's ESLINT rules nested ternary operators are forbidden, so I have to get rid of the above.

I'm trying to find out alternatives to this approach. I really don't want to turn it into a huge if / else statement, but don't know if there's any other options.

Answer

Your alternatives here are basically:

  1. That if/else you don't want to do
  2. A switch combined with if/else

I tried to come up with a reasonable lookup map option, but it got unreasonable fairly quickly.

I'd go for #1, it's not that big:

if (res.distance == 0) {
    word = 'a';
} else if (res.distance == 1 && res.difference > 3) {
    word = 'b';
} else if (res.distance == 2 && res.difference > 5 && String(res.key).length > 5) {
    word = 'c';
} else {
    word = 'd';
}

If all the braces and vertical size bother you, without them it's almost as concise as the conditional operator version:

if (res.distance == 0) word = 'a';
else if (res.distance == 1 && res.difference > 3) word = 'b';
else if (res.distance == 2 && res.difference > 5 && String(res.key).length > 5) word = 'c';
else word = 'd';

(I'm not advocating that, I never advocate leaving off braces or putting the statement following an if on the same line, but others have different style perspectives.)

#2 is, to my mind, more clunky but that's probably more a style comment than anything else:

word = 'd';
switch (res.distance) {
    case 0:
        word = 'a';
        break;
    case 1:
        if (res.difference > 3) {
            word = 'b';
        }
        break;
    case 2:
        if (res.difference > 5 && String(res.key).length > 5) {
            word = 'c';
        }
        break;
}

And finally, and I am not advocating this, you can take advantage of the fact that JavaScript's switch is unusual in the B-syntax language family: The case statements can be expressions, and are matched against the switch value in source code order:

switch (true) {
    case res.distance == 0:
        word = 'a';
        break;
    case res.distance == 1 && res.difference > 3:
        word = 'b';
        break;
    case res.distance == 2 && res.difference > 5 && String(res.key).length > 5:
        word = 'c';
        break;
    default:
        word = 'd';
        break;
}

How ugly is that? :-)