Logical Functions

Jess has no logical functions. Conditional evaluation is a language form, $if / $else, not something you import from @jesscss/fns.

iif() is retired

Pre-2.x Jess spelled conditionals iif(cond, a, b) and @include $iif(cond, mixin());. That syntax no longer exists: iif is not registered in the Jess built-in function set, and @let / @mixin / @include are not Jess at-rules. Use $if / $else instead. (The iif() helper still ships in @jesscss/fns for Less sources — it is Less's if(), not a Jess API.)

$if / $else

$value: 20;

.box {
  $if ($value > 10) { width: 20px; } $else { width: 10px; }
}

Outputs:

.box {
  width: 20px;
}

Guard-style dispatch

This is the Jess equivalent of Less's when guards with a default() fallback: nested mixin definitions selected by $if.

one($value) {
  one_1() {
    width: 2px;
  }
  one_2() {
    height: 10px;
  }
  def() {
    width: 4px;
  }

  $if ($value > 1) { $ > one_1(); }
  $if ($value > 5) { $ > one_2(); }
  $if (not ($value > 1) and not ($value > 5)) { $ > def(); }
}

.box-0 {
  $ > one(0);
}
.box-2 {
  $ > one(2);
}
.box-10 {
  $ > one(10);
  $if (true) { color: #333; } $else { color: #555; }
  $if (false) { background-color: #333; } $else { background-color: #555; }
}

This produces:

.box-0 {
  width: 4px;
}
.box-2 {
  width: 2px;
}
.box-10 {
  width: 2px;
  height: 10px;
  color: #333;
  background-color: #555;
}

info

Logical operators are condition-position forms: write not, and, and or inside a $if (…) header rather than !, &&, and ||.

See Conditionals & iteration for $else if, comparison operators, and scoping rules.