isdefined not working as intended

[macookinerbarry-png]

problems with using isdefined()

I'm trying to use "isdefined()" in a way where I have an if() block to check if the variable is defined, and if the condition returns true, I want to return the value of the variable. But no matter what I do, "isdefined()" either returns false or the complete string: "isdefined(@variable)";

Current behavior:

@a: blue;
@b: if((isdefined(@a)), @a, black);

@b = black

Expected behavior:

@a: blue;
@b: if((isdefined(@a)), @a, black);

@b = blue

also

Current behavior:

@b: isdefined(@a);

@b = false

✅ which is correct

@a:red;
@b: isdefined(@a);

@b = isdefined(red);

❌ .....should be true

Also, not defining @a will always throw an error "@A is undefined"

The funny thing is this works perfectly fine when trying to use this on the less playground.

Environment information:
I tested this in VS code using the CDN links to compile LESS

• less version: 4.4.2
• operating system: windows

[dosubot]

Hi @macookinerbarry-png! I'm Dosu and I’m helping the less.js team.

The isdefined() function in LESS expects a variable node as its argument and tries to evaluate it in the current context. If you pass a direct value (like a color keyword), it won't return true or false as expected—instead, you'll get the unevaluated string or false, because 'red' isn't a variable node. This is by design in the LESS implementation (source).

Your issue is likely caused by differences in how the LESS playground and your local environment (VS Code with CDN) handle evaluation and parsing. LESS variable evaluation and function calls happen at parse time, not execution time, which can affect dynamic checks like variable existence (source). Also, a regression in LESS 4.4.2 related to CSS function handling may affect isdefined() (source).

To reliably check if a variable is defined, always pass a variable node (e.g., @A) to isdefined(), not a direct value. There isn't an official workaround for checking if a value is defined as a variable, but you can avoid passing literals to isdefined() and structure your logic to only check actual variables. If you need more dynamic checks, LESS's current architecture may not support this due to its parse-time evaluation model.

To reply, just mention @dosu.

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[puckowski]

This Less:

@a: blue;
@b: if((isdefined(@a)), @a, black);

div {
  color: @b;
}

@aa:red;
@ba: isdefined(@aa);

div {
  visible: @ba;
}

works for me on the playground. It also works for me using 4.4.2 in the browser in a fresh project and works using 4.4.2 lessc in a fresh project.

I get:

div {
  color: blue;
}
div {
  visible: true;
}

It is possible you have a more complex scenario where @a exists but is not in the same evaluation scope as @b. If the scope is different you would see black as the output for

@b: if((isdefined(@a)), @a, black);

Do you have any other hints about your project setup? Do you have any additional CSS processing steps that may be interfering with Less?

[matthew-dean]

Confirmed that this code as written works today.