What causes "`Consts must have a value: '${constLine}'`" in BabylonJS/Babylon.js?

Add a default value to the const: `const float exposure = 1.0;`.

BabylonJS/Babylon.js · error · Error

`Consts must have a value: '${constLine}'`

Error message

`Consts must have a value: '${constLine}'`

What it means

The parsed const line has a name but the default-value capture is null. Property consts must carry a default value because it becomes the initial value of the generated Smart Filter property. This is the 'value missing' variant of the const validation errors.

Source

Thrown at packages/dev/smartFilters/src/utils/buildTools/shaderConverter.ts:225

        const constLine = matches[2];

        if (!constLine) {
            throw new Error("Const line not found");
        }

        const constLineMatches = new RegExp(/^const\s+(\w+)\s+(\w+)\s*=\s*([^\s;]+)\s*;?\s*$/gm).exec(constLine);
        if (!constLineMatches || constLineMatches.length < 4) {
            throw new Error(`Consts must have a name, type, and a default value: '${constLine}'`);
        }
        const type = constLineMatches[1];
        const friendlyName = constLineMatches[2];
        const defaultValue = constLineMatches[3];

        if (!friendlyName) {
            throw new Error(`Consts must have a name: '${constLine}'`);
        }
        if (defaultValue === null) {
            throw new Error(`Consts must have a value: '${constLine}'`);
        }

        const constProperty: ConstPropertyMetadata | null =
            type === "float"
                ? {
                      name: DecorateSymbol(friendlyName),
                      friendlyName,
                      type,
                      defaultValue: parseFloat(defaultValue),
                      options: annotation.property.options as ConstMetadataAnnotationFloatOptions | undefined,
                  }
                : null;

        if (!constProperty) {
            throw new Error(`Unsupported const property type: '${type}'`);
        }
        fragmentConstProperties.push(constProperty);

View on GitHub (pinned to 0592b347b8)

Solutions

Add a default value to the const: `const float exposure = 1.0;`.
If no default is desired, remove the {"property":...} annotation so the const is treated as ordinary.
Set the default via the annotation's property options if a specific initial value is needed (the defaultValue still must be syntactically present).

Example fix

// before
// {"property": {"type": "float"}}
const float exposure;
// after
// {"property": {"type": "float"}}
const float exposure = 1.0;

Defensive patterns

Strategy: validation

Validate before calling

if (/const\s+\w+\s+\w+\s*;/.test(shader)) throw new Error("Property consts must declare a default value");

Type guard

function constHasValue(line: string): boolean {
  const m = /^const\s+\w+\s+\w+\s*=\s*([^\s;]+)/.exec(line);
  return !!m && m[1] !== null && m[1].length > 0;
}

Try / catch

try {
  const info = ParseFragmentShader(blockName, namespace, shader);
} catch (e) {
  if (e instanceof Error && e.message.startsWith("Consts must have a value")) {
    // add an initializer to the const and retry
  } else throw e;
}

Prevention

Always initialize consts at declaration (required by property consts).
If a const is computed at runtime, use a uniform instead of a const.
Grep shader assets for `const \w+ \w+;` before building.

When it happens

Trigger: ParseFragmentShader on `const float exposure;` or `const float exposure = ;` — an annotated property const declared without an initializer.

Common situations: Declaring a const intending to assign later (invalid for property consts); refactoring removed the initializer; template-generated shader with a missing value placeholder.

Related errors

AI-assisted analysis of BabylonJS/Babylon.js@0592b347b8 (2026-08-30). Data as JSON: /api/errors/6811fd7ec722a49d. Report an issue: GitHub.