LESS - How to use & when, if conditions in less css [duplicate] - less

Question

I'm looking for some kind of if-statement to control the background-color of different div elements.
I have tried the below, but it doesn't compile
#debug: true;
header {
background-color: (yellow) when (#debug = true);
#title {
background-color: (orange) when (#debug = true);
}
}
article {
background-color: (red) when (#debug = true);
}

Answer 1

There is a way to use guards for individual (or multiple) attributes.
#debug: true;
header {
/* guard for attribute */
& when (#debug = true) {
background-color: yellow;
}
/* guard for nested class */
#title when (#debug = true) {
background-color: orange;
}
}
/* guard for class */
article when (#debug = true) {
background-color: red;
}
/* and when debug is off: */
article when not (#debug = true) {
background-color: green;
}
...and with Less 1.7; compiles to:
header {
background-color: yellow;
}
header #title {
background-color: orange;
}
article {
background-color: red;
}

Answer 2

LESS has guard expressions for mixins, not individual attributes.
So you'd create a mixin like this:
.debug(#debug) when (#debug = true) {
header {
background-color: yellow;
#title {
background-color: orange;
}
}
article {
background-color: red;
}
}
And turn it on or off by calling .debug(true); or .debug(false) (or not calling it at all).

Answer 3

I stumbled over the same question and I've found a solution.
First make sure you upgrade to LESS 1.6 at least.
You can use npm for that case.
Now you can use the following mixin:
.if (#condition, #property, #value) when (#condition = true){
#{property}: #value;
}
Since LESS 1.6 you are able to pass PropertyNames to Mixins as well. So for example you could just use:
.myHeadline {
.if(#include-lineHeight, line-height, '35px');
}
If #include-lineheight resolves to true LESS will print the line-height: 35px and it will skip the mixin if #include-lineheight is not true.

Answer 4

I wrote a mixin for some syntactic sugar ;)
Maybe someone likes this way of writing if-then-else better than using guards
depends on Less 1.7.0
https://github.com/pixelass/more-or-less/blob/master/less/fn/_if.less
Usage:
.if(isnumber(2), {
.-then(){
log {
isnumber: true;
}
}
.-else(){
log {
isnumber: false;
}
}
});
.if(lightness(#fff) gt (20% * 2), {
.-then(){
log {
is-light: true;
}
}
});
using on example from above
.if(#debug, {
.-then(){
header {
background-color: yellow;
#title {
background-color: orange;
}
}
article {
background-color: red;
}
}
});