## Problem Build a JavaScript/TypeScript form validation library. Each field has one or more validation rules. Rules are composable — validators can be chained. When `validate()` is called, return all field errors. ```typescript type Rule = (value: string) => string | null; // null = pass, string = error message class FormValidator { addField(name: string, ...rules: Rule[]): this; validate(data: Record ): Record ; // field -> errors } // Built-in rules const required: Rule; const minLength = (n: number): Rule => ...; const matches = (pattern: RegExp, msg: string): Rule => ...; ``` ## Example ```typescript const validator = new FormValidator() .addField("email", required, matches(/@/, "must contain @")) .addField("password", required, minLength(8)); validator.validate({ email: "notanemail", password: "short" }); // -> { email: ["must contain @"], password: ["must be at least 8 characters"] } validator.validate({ email: "user@x.com", password: "securepass" }); // -> {} ``` ## Follow-ups 1. How do you support async validators (e.g., checking if a username is taken via API)? 2. How do you implement cross-field validation (e.g., "password" must match "confirm_password")? 3. How would you integrate this with React controlled components?

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Client Side Validation: Build a Form Validation Library with Composable Rules

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Problem Build a JavaScript/TypeScript form validation library. Each field has one or more validation rules. Rules are composable — validators can be chained. When validate() is called, return all field errors. Example Follow-ups How do you support async validators (e.g., checking if a username is taken via API)? How do you implement cross-field validation (e.g., "password" must match "confirm_password")? How would you integrate this with React controlled components?

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Client Side Validation: Build a Form Validation Library with Composable Rules

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