Design a simple templating system for ad copy generation

WHAT THIS TESTS: This question evaluates whether you can draw a clean boundary between data and presentation. A template processor is software designed to combine templates with data to produce resulting documents or programs, so the interviewer wants to see you treat the ad copy generator as a small but complete template system rather than a one-off string concatenation hack.

A GOOD ANSWER COVERS: First, define a typed data model or schema that captures product name, price as a structured money object, and call to action, keeping the data layer separate from the template language. Second, choose a lightweight template syntax such as double curly braces and decide whether templates live as strings, files, or compiled functions. Third, describe a rendering pipeline with at least two stages, parsing the template into tokens and then merging those tokens with the data model to produce the formatted output. Fourth, handle missing variables explicitly through defaults, empty strings, or validation errors rather than silent undefineds. Fifth, flag security and formatting concerns, price should be locale formatted, not raw concatenation, and if the result document is a web page or HTML fragment, user supplied fields must be escaped to prevent injection.

COMMON WRONG ANSWERS: Proposing a single function that takes three strings and returns a JavaScript template literal with no schema or validation. Hardcoding English word order so the system cannot support languages with different grammar structures. Treating price as a raw float string, which destroys currency formatting and invites rounding bugs. Ignoring that a template engine is ordinarily included as part of a larger system, so it should expose a clean API rather than global state.

LIKELY FOLLOW-UPS: How would you add conditionals, for example showing a strikeout price only when a discount exists? How would you cache compiled templates when generating millions of ads? How would you support localization where word order changes across languages? What happens if a marketer accidentally passes HTML in a product name?

ONE CONCRETE EXAMPLE: A minimal but solid design uses a JSON data object with fields productName, priceCents, currency, and ctaLabel. The template engine accepts a string like Buy {{productName}} for {{price}}. {{cta}}. The engine tokenizes the string, looks up each key in the data map, runs price through an i18n formatter, HTML-escapes productName, and returns the final string. Compiled templates are cached in a Map keyed by template hash so repeated renders are O of N on output length, not parsing cost.