TypeScript object-key guide
Convert Object Keys to camelCase in TypeScript
TypeScript applications often need both a runtime transformation and a matching compile-time shape when snake_case API payloads become camelCase frontend objects.
TypeScript implementation
type CamelCase<S extends string> =
S extends `${infer Head}_${infer Tail}`
? `${Lowercase<Head>}${Capitalize<CamelCase<Tail>>}`
: Uncapitalize<S>
type Camelize<T> =
T extends readonly (infer Item)[] ? Camelize<Item>[] :
T extends Record<string, unknown> ? { [K in keyof T as K extends string ? CamelCase<K> : K]: Camelize<T[K]> } : T
function toCamelKey(key: string) {
return key.replace(/[-_]+([a-zA-Z0-9])/g, (_, char: string) => char.toUpperCase())
}
function keysToCamel<T>(value: T): Camelize<T> {
if (Array.isArray(value)) return value.map(keysToCamel) as Camelize<T>
if (value === null || typeof value !== "object") return value as Camelize<T>
const output: Record<string, unknown> = {}
for (const [key, child] of Object.entries(value)) {
const nextKey = toCamelKey(key)
if (Object.hasOwn(output, nextKey)) throw new Error(`Key collision: ${nextKey}`)
output[nextKey] = keysToCamel(child)
}
return output as Camelize<T>
}A recursive mapped type can describe remapped keys and array elements so callers get a useful transformed type.
The runtime function still performs the real conversion; type-level remapping cannot change network data.
Production edge cases
- Complex acronym rules can make runtime and type-level transformations diverge.
- Key collisions remain a runtime data problem even when the TypeScript type looks valid.
- Symbol keys and non-JSON class instances need explicit handling.
- If semantic prefixes are preserved at runtime, model those exceptions in types when exact inference matters.
Test object keys online
Paste a JSON object to test recursive key conversion, top-level-only mode, excluded keys, underscore/dollar prefixes, and collision handling before you copy the logic into an application.
Drop a .json file here or choose one from your device
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{
"userProfile": {
"firstName": "Ada",
"createdAt": "2026-09-12"
}
}Parsing, file reading, formatting, and key conversion run locally in your browser. JSON input is not sent to a conversion server.