GitbookIO/gitbook · error · ExpressionError

Empty or invalid expression

Error message

Empty or invalid expression

What it means

Thrown by ExpressionRuntime.parse in @gitbook/expr when the parsed program has no body statements (or an empty one), meaning the expression is empty or wholly unparseable. Both the strict and loose parsers returned an AST with nothing executable in it, so downstream evaluation or variable extraction is impossible.

Source

Thrown at packages/expr/src/runtime.ts:202

        }
    }

    /**
     * Parses a binary expression and returns an @ExpressionParserResult.
     */
    public parse(
        expr: string,
        options: { loose?: boolean } = {
            loose: false,
        }
    ): ExpressionParserResult {
        try {
            const ast = options.loose
                ? parseLoose(expr, { ...this.#parserOptions })
                : parse(expr, { ...this.#parserOptions });

            if (!ast.body || ast.body.length === 0) {
                throw new ExpressionError('Empty or invalid expression');
            }

            // Extract the first expression statement that we find
            const firstExprIndex = ast.body.findIndex((node) => isParsedExpressionStatement(node));
            const [statement] = ast.body.splice(firstExprIndex, 1);

            if (!statement || !isParsedExpressionStatement(statement)) {
                throw new ExpressionError('Empty or invalid expression');
            }

            // Return information on the other nodes as invalid nodes
            const invalidNodes = ast.body.filter(filterOutModuleDeclarationStatement);

            return {
                result: statement.expression,
                invalidNodes,
            };
        } catch (error) {

View on GitHub (pinned to db67585ee2)

Solutions

1. Skip evaluation when the expression is empty/whitespace-only (early return a default)
2. Default missing expression fields to a sensible literal like 'true' or 'null' depending on context
3. Trim and validate input in the editor/UI before submitting to the runtime

Example fix

// before
const value = runtime.evaluate(expr /* '' */, inputs);

// after
const value = expr.trim() ? runtime.evaluate(expr, inputs) : true;

if (!expr.trim()) {
    return true; // sensible default instead of parsing
}
const value = runtime.evaluate(expr, inputs);

function isNonEmptyExpression(expr: string): boolean {
    return expr.trim().length > 0;
}

When it happens

Trigger: Calling parse/evaluate/getVariables with an empty string, a whitespace-only string, or a string consisting only of comments or characters the parser drops entirely.

Common situations: Optional expression fields left blank ('' or undefined stringified); trimming user input down to nothing before passing it; building expressions dynamically so an empty fragment reaches the runtime.

Related errors

• Invalid nodes found when parsing
• Expected schema of object to be provided: ${schema.type}
• Unsupported schema type: ${schema.type}

AI-assisted analysis of GitbookIO/gitbook@db67585ee2 (2026-08-28). Data as JSON: /api/errors/5b2023498127c3dc. Report an issue: GitHub.