What causes ""Const line not found"" in BabylonJS/Babylon.js?

Ensure every // {"property":...} annotation is immediately followed (next line) by a full `const = ;` declaration.

BabylonJS/Babylon.js · error · Error

"Const line not found"

Error message

"Const line not found"

What it means

ParseFragmentShader in the smartFilters shader converter throws this while iterating const declarations that carry a {"property":...} annotation. The regex matched a 'const' group, but capture group 2 (the const line text itself) is empty. It is a defensive invariant check — it almost always means the annotation/const pairing in the shader source is malformed rather than a real empty const line.

Source

Thrown at packages/dev/smartFilters/src/utils/buildTools/shaderConverter.ts:210

    for (const matches of constGroups) {
        const annotationJSON = matches[1];

        // If it doesn't have any annotation, treat it as a regular const
        if (!annotationJSON) {
            continue;
        }

        const annotation = JSON.parse(annotationJSON.replace("//", "").trim()) as ConstMetadataAnnotation;

        // If the annotation doesn't have a "property" field, treat it as a regular const
        if (!annotation.property) {
            continue;
        }

        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 =

View on GitHub (pinned to 0592b347b8)

Solutions

Ensure every // {"property":...} annotation is immediately followed (next line) by a full `const <type> <name> = <value>;` declaration.
Keep the annotation and its const in the same block and do not run code strippers/minifiers on the shader source before conversion.
If the const should not be exposed as a property, remove the 'property' field from the annotation (or the whole annotation) so the parser skips it.
Add the const line back explicitly, e.g. `const float myVal = 0.5;` right after the annotation.

Example fix

// before
// {"property": {"type": "float", "options": {...}}}

float unrelated = 1.0;
// after
// {"property": {"type": "float", "options": {...}}}
const float brightness = 0.5;

Defensive patterns

Strategy: validation

Validate before calling

const annotated = shader.match(/\/\/\s*\{.*\}\s*\n\s*const .*/g);
if (!annotated || annotated.some(m => !/^\/\/\s*\{.*\}\s*\n\s*const \w+ \w+ = [^\s;]+;?\s*$/m.test(m))) {
  throw new Error("Every property annotation must be immediately followed by a single-line const declaration");
}

Type guard

function hasConstAfterAnnotation(shader: string): boolean {
  return [...shader.matchAll(/(\/\/\s*\{.*\}\s*(?:\r?\n)+)(const .*)/gm)].every(m => typeof m[2] === "string" && m[2].length > 0);
}

Try / catch

try {
  const info = ParseFragmentShader(blockName, namespace, shader);
} catch (e) {
  if (e instanceof Error && e.message === "Const line not found") {
    // fix the annotation/const pairing in the shader source and retry once
  } else throw e;
}

Prevention

Always place property annotations on the line directly above their const declaration.
Never run minifiers or code strippers on shader source before ParseFragmentShader.
Keep a unit test that converts every shipped shader at build time.
Validate shaders with a lint rule that pairs // {...} annotations with following const lines.

When it happens

Trigger: Calling ParseFragmentShader (via fragmentShaderInfo/result) on a fragment shader where a // {"property":...} annotation is present but the immediately following const declaration cannot be captured by the regex /(\/\/\s*\{.*\}\s*(?:\r\n|\r|\n)+)?(const .*)/gm (e.g. annotation separated from a 'const' keyword by intervening code, or the const text was mangled by preprocessing).

Common situations: Hand-edited shaders where the annotation comment and the const line got separated or merged; shader strings built programmatically with wrong line endings; minified/stripped shaders where the const line was removed but the annotation comment left behind; copy-paste of annotations without their const.

Related errors

AI-assisted analysis of BabylonJS/Babylon.js@0592b347b8 (2026-08-30). Data as JSON: /api/errors/81ea22ee2a2a26b8. Report an issue: GitHub.