Introduction

Rollup provides efficient JavaScript bundling with ES module support, but incorrect configurations, improper dependency resolution, and misconfigured plugins can lead to bloated output and runtime failures. Common pitfalls include failing to mark external dependencies correctly, using improper tree-shaking strategies, and incorrectly configuring output formats. These issues become particularly problematic in large-scale applications where bundle size and load performance are critical. This article explores advanced Rollup troubleshooting techniques, performance optimization strategies, and best practices.

Common Causes of Unexpected Bundle Issues in Rollup

1. Inefficient Tree Shaking Resulting in Large Bundles

Failing to remove unused code increases the final bundle size.

Problematic Scenario

// Importing entire library instead of specific functions
import * as lodash from "lodash";
console.log(lodash.shuffle([1, 2, 3]));

Importing entire libraries prevents Rollup from tree shaking unused functions.

Solution: Use Named Imports

// Optimized import
import { shuffle } from "lodash";
console.log(shuffle([1, 2, 3]));

Using named imports ensures only necessary functions are included.

2. Plugin Misconfiguration Leading to Build Failures

Incorrect plugin usage results in errors during the bundling process.

Problematic Scenario

// Incorrect use of rollup-plugin-commonjs
import commonjs from "@rollup/plugin-commonjs";
export default {
    input: "src/index.js",
    output: {
        file: "dist/bundle.js",
        format: "esm"
    },
    plugins: [commonjs()]
};

Using `@rollup/plugin-commonjs` in an ES module build can cause issues.

Solution: Use `external` and Proper Plugin Configuration

// Optimized Rollup configuration
import commonjs from "@rollup/plugin-commonjs";
import resolve from "@rollup/plugin-node-resolve";
export default {
    input: "src/index.js",
    output: {
        file: "dist/bundle.js",
        format: "esm"
    },
    external: ["lodash"],
    plugins: [resolve(), commonjs()]
};

Marking dependencies as `external` prevents unnecessary bundling.

3. Improper Code Splitting Resulting in Large Entry Bundles

Failing to split code efficiently leads to excessive initial load times.

Problematic Scenario

// Bundling everything into a single file
export default {
    input: "src/main.js",
    output: {
        file: "dist/bundle.js",
        format: "esm"
    }
};

Including all code in one file increases bundle size and slows execution.

Solution: Implement Code Splitting

// Optimized configuration with code splitting
export default {
    input: {
        main: "src/main.js",
        vendor: "src/vendor.js"
    },
    output: {
        dir: "dist",
        format: "esm"
    }
};

Separating vendor code from application logic improves performance.

4. Circular Dependencies Causing Infinite Loops

Modules that import each other create circular dependencies, causing runtime errors.

Problematic Scenario

// Circular dependency between modules
// fileA.js
import { funcB } from "./fileB.js";
export function funcA() { return funcB(); }

// fileB.js
import { funcA } from "./fileA.js";
export function funcB() { return funcA(); }

Mutual imports cause Rollup to enter an infinite loop.

Solution: Refactor to Break Circular Dependencies

// Extract shared logic into a separate module
// shared.js
export function sharedFunction() { return "Shared Logic"; }

// fileA.js
import { sharedFunction } from "./shared.js";
export function funcA() { return sharedFunction(); }

// fileB.js
import { sharedFunction } from "./shared.js";
export function funcB() { return sharedFunction(); }

Extracting shared logic into a separate module resolves circular dependencies.

5. External Dependencies Being Bundled Unexpectedly

Rollup sometimes includes external libraries in the bundle instead of treating them as separate dependencies.

Problematic Scenario

// Expecting lodash to be external, but it is bundled
export default {
    input: "src/index.js",
    output: {
        file: "dist/bundle.js",
        format: "esm"
    }
};

Without specifying `external`, Rollup includes lodash in the output.

Solution: Mark Dependencies as External

// Optimized configuration with external dependencies
export default {
    input: "src/index.js",
    output: {
        file: "dist/bundle.js",
        format: "esm"
    },
    external: ["lodash"]
};

Marking libraries as `external` reduces bundle size.

Best Practices for Optimizing Rollup Performance

1. Use Named Imports Instead of Whole Library Imports

Ensure tree shaking removes unused code by using named imports.

2. Configure Plugins Correctly

Use `@rollup/plugin-commonjs` and `@rollup/plugin-node-resolve` in the correct order.

3. Implement Code Splitting

Split large modules into separate chunks to optimize initial load time.

4. Resolve Circular Dependencies

Refactor modules to break circular dependencies by extracting shared logic.

5. Mark External Dependencies Explicitly

Use `external` in the Rollup configuration to avoid unnecessary bundling.

Conclusion

Rollup applications can suffer from large bundle sizes, build failures, and runtime errors due to improper tree shaking, plugin misconfiguration, circular dependencies, and unexpected inclusion of external dependencies. By optimizing import strategies, configuring plugins properly, implementing code splitting, resolving circular dependencies, and marking external modules correctly, developers can significantly improve Rollup performance. Regularly analyzing bundle output using `rollup-plugin-analyzer` and testing builds with `--config-debug` helps detect and resolve inefficiencies proactively.