Sass meta.function-exists() Function

What You’ll Learn

meta.function-exists() belongs to the built-in sass:meta module. It answers a simple question: is there a function named X? This page covers $module with @use, built-ins vs custom functions, safer alternatives to deprecated meta.feature-exists, and five compiled examples.

What Is meta.function-exists()?

Official docs: returns whether a function named $name is defined—either a built-in or a user-defined @function. Use it when a library or theme wants an optional helper without crashing if that helper is missing.

• Without $module, Sass looks for a function in the current global / local scope (including many built-ins).
• With $module, it looks inside a @use namespace from the current file.
• Missing names return false—they do not throw.

📝 Syntax

@use "sass:meta";

// Recommended
meta.function-exists($name, $module: null)

// Legacy global name (no $module argument)
function-exists($name)

Parameters

Return value

📦 Loading sass:meta

// Recommended — namespaced
@use "sass:meta";
@use "sass:math";
$ok: meta.function-exists("pow", $module: "math");

// Optional — bring members into scope
@use "sass:meta" as *;
$ok: function-exists("lighten");

// Legacy global
$ok: function-exists("lighten");

🔗 Using the $module Argument

After @use "sass:math", functions like pow live under the math namespace. Official docs check them with a module name:

@use "sass:meta";
@use "sass:math";

// Official style (positional $module)
@debug meta.function-exists("div", "math"); // true

// Equivalent named argument
@debug meta.function-exists("div", $module: "math"); // true

// Without $module, "div" is not a free global name in modern Sass
@debug meta.function-exists("div"); // false

$module must match the namespace used in the current file. If you write @use "sass:math" as m, pass $module: "m".

📋 Related Meta Helpers

🛠 Compatibility

This is about Sass compilers, not browsers. The lookup runs at compile time.

Prefer current Dart Sass so @use namespaces work as shown in the docs.

Examples Gallery

Each example uses meta.function-exists at compile time. Open View Compiled CSS for verified Dart Sass output.

@use "sass:meta";

.before {
  --add: #{meta.function-exists("add")};
}

@function add($a, $b) {
  @return $a + $b;
}

.after {
  --add: #{meta.function-exists("add")};
  width: add(20px, 20px);
}

.before {
  --add: false;
}

.after {
  --add: true;
  width: 40px;
}

@use "sass:meta";
@use "sass:math";

.probe {
  --math-div: #{meta.function-exists("div", $module: "math")};
  --math-pow: #{meta.function-exists("pow", $module: "math")};
  --bare-div: #{meta.function-exists("div")};
}

.probe {
  --math-div: true;
  --math-pow: true;
  --bare-div: false;
}

@use "sass:meta";
@use "sass:math";

@function scale-size($n) {
  @if meta.function-exists("pow", $module: "math") {
    @return math.pow($n, 2) * 1px;
  }
  @return $n * $n * 1px;
}

.box {
  width: scale-size(4);
}

.box {
  width: 16px;
}

@use "sass:meta";

@function double($n) {
  @return $n * 2;
}

$name: "double";

.box {
  @if meta.function-exists($name) {
    width: meta.call(meta.get-function($name), 48px);
  } @else {
    width: 48px;
  }
}

.box {
  width: 96px;
}

@use "sass:meta";
@use "sass:math";

.flags {
  --nope: #{meta.function-exists("definitely-missing")};
  --typo-pow: #{meta.function-exists("poww", $module: "math")};
  --scale-color: #{meta.function-exists("scale-color")};
}

.flags {
  --nope: false;
  --typo-pow: false;
  --scale-color: true;
}

🚀 Real-World Use Cases

• Optional theme hooks — call a user-provided @function only if it exists.
• Module-aware libraries — detect sass:math / sass:color helpers via $module.
• Safe dynamic APIs — guard meta.get-function + meta.call.
• Migration off feature-exists — preferred modern detection for function APIs.
• Plugin registries — enable features when a named helper is present.

⚠️ Common Pitfalls

• Forgetting $module — math.div will not match a bare "div" check.
• Wrong namespace string — must match the @use … as name in this file.
• Confusing with get-function — exists returns a boolean; get-function returns a value.
• Checking mixins — use meta.mixin-exists instead.
• Expecting runtime checks — everything resolves when Sass compiles.

💡 Best Practices