What is downleveling?

0-2 years experience

1. 1We need a new component that must run in IE11. How would you set up tsconfig so the emitted JavaScript is compatible?
2. 2If you add optional chaining to a file that’s compiled for ES5, what will the output look like and why?

2-5 years experience

1. 1Our TypeScript library is published with a target of ES2020, but some consumers on Node 10 are seeing syntax errors. Walk me through how you’d debug and fix the downleveling issue.
2. 2Explain why a piece of code using async/await throws at runtime after compilation to ES5, and what steps you’d take to resolve it.

5-8 years experience

1. 1In a monorepo, some packages need to downlevel to ES5 for a legacy web app while others can stay at ES2022. How would you design the build pipeline to handle these mixed targets efficiently?
2. 2Discuss the trade‑offs between downleveling async/await to generator helpers versus pulling in a runtime polyfill for a large single‑page application.

8+ years experience

1. 1We’re planning a multi‑year migration from an ES5 front‑end to a modern ES2022 stack. What architectural guidelines would you establish for downleveling strategy, versioning, and CI/CD to keep backward compatibility while gradually retiring polyfills across teams?
2. 2How would you coordinate cross‑team decisions about downleveling targets to avoid version drift in a large organization with both legacy and cutting‑edge products?