Coerce null and undefined to empty string with Zod

I love using zod for parsing in TypeScript. Getting Zod to default to empty string when parsing null or undefined is a bit...hard.

Luckily, it is easy to write extensions for it:

// This class has other extensions
export class zx {
  static string(): z.ZodEffects<z.ZodString, string, unknown> {
    return z.preprocess((arg) => {
      if (arg === null || arg === undefined) {
        return "";
      }

      if (typeof arg === "string") {
        return arg;
      }
      if (typeof arg === "number") {
        return arg.toString();
      }

      return "";
    }, z.string());
  }
}

It correctly parses undefined and null as ''.

Primary use case is to force form values to be initialized to the empty string instead of using undefined or null when binding to <input>.