Set LESS variable if it's not set

Question

Is it possible to set a LESS variable to a given value, if it hasn't been defined?

I can only seem to find solutions for SASS.

Answer 1

An important remark: it turns out that the need for such trick very often happens to be an XY-Problem (as result of misunderstanding Less variable semantics). Normally there's no real use-cases where you actually want this (in Less you instead always define a default value (so it simply always exists) to be overridden at will). So before using the trick below, make sure it's not something else you want actually.

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It is quite simple (since v1.4.0) if you need to provide such variable for the same scope (i.e. global or local) where it could already be defined:

@bar: 41; // comment/uncomment to test

.define-bar-if-not-defined() {@bar: 42}
.define-bar-if-not-defined(); // exposes a variable only if it's not already in this scope

#using-global-bar {
    global-bar: @bar;
}

#using-local-bar {
    @bar: 43; // comment/uncomment to test
    .define-bar-if-not-defined();

    local-bar: @bar;
}

However this method won't work if you try to use it for global/parent scope variables used in local scope (and that may be a problem if you need a generic method when you're not sure what scope the original variable could possibly come from):

#using-unknown-bar-in-local-scope {
    // ... some code ...

    .define-bar-if-not-defined(); // won't override local @bar (if any) but always overrides global and parent scope @bar

    bar: @bar;
}

In that case I don't know any straight-forward solution (w/o some inline javascript hacks one could use in guard expression for example)