React interview questions

Write mergeDefaultProps(props, defaults). It returns a new plain object that has every key from defaults, overridden by any key present in props whose value is not undefined. Neither input object should be mutated. This is the logic behind default props: a caller who does not

Write buildClassName(base, conditional). base is a string (possibly with several space-separated classes). conditional is an object mapping class name to a boolean. Return a single space-separated class string containing base's classes followed by every key of conditional whose

Write flattenChildren(children). children is a JSON value shaped like props.children can be: a single value, or arrays nested to any depth (because {list.map(...)} and adjacent expressions produce nested arrays). Return a flat array containing every leaf value in order, except

Write validateRequiredProps(schema, props). schema is an object mapping prop name to a spec { required: boolean, type: "string" | "number" | "boolean" | "function" | "object" }. props is the object actually passed to the component. Return a sorted array of human-readable

In plain terms: what is a React component, and what are props? Give me a concrete example of a parent handing something to a child.

You write <Card title="Hi">text</Card> in a .jsx file. What does that line actually turn into when the code runs, and why does a component have to return a single root node?