Conditionally setting one variable's value based on another

Question

I've got a Less variable called @side. What I want is to set the variable @sideOpposite depending on the value of the @side variable. It can take only two values: "left" or "right".

In other words I need a Less equivalent of the JS code:

var side = "left",
    sideOpposite = (side === "left")? "right" : "left";

I've tried to accomplish this using when function, but from what I understand it doesn't work that way and is only applicable to CSS properties, not variables:

when (@side = right){
  @sideOpposite: left;
}
when (@side = left){
  @sideOpposite: right;
}

Answer 1

See Mixin Guards and Ruleset Guards (aka "CSS Guards") for when usage examples. Since you need to define a variable you'll have to use mixin-based condition (rulesets do not expose their variables to an outer scope). E.g.:

.-();
.-() when (@side = right) {
    @sideOpposite: left;
}
.-() when (@side = left) {
    @sideOpposite: right;
}

(Use any suitable mixin name instead of .-, e.g. .define-opposite-side).

---

And with Argument Pattern Matching it can be further simplified just to:

.-(@side); 
.-(left)  {@sideOpposite: right}
.-(right) {@sideOpposite: left}