URL Encode / Decode — TypeScript source

// Pure surface of the url-encode tool (slug: url-encode). The operations are
// the four platform builtins, but pinning the mode × scope matrix here with
// safe-character vectors defines what a Go twin must reproduce — Go's
// net/url escaping differs (QueryEscape uses '+' for spaces; JS always uses
// %20; the unreserved sets do not match).
export type UrlCodeMode = 'encode' | 'decode';
export type UrlScope = 'component' | 'uri';

export type UrlTransformResult =
  | { output: string; error: null }
  | { output: ''; error: null } // empty input
  | { output: ''; error: string };

/** Encode/decode under the component scope (encodeURIComponent — everything
 *  but A-Z a-z 0-9 - _ . ! ~ * ' ( )) or the full-URI scope (encodeURI —
 *  additionally preserves ; / ? : @ & = + $ , #). Malformed % sequences on
 *  decode surface the platform URIError message. */
export function transformUrl(
  input: string,
  mode: UrlCodeMode,
  scope: UrlScope,
): UrlTransformResult {
  if (!input) return { output: '', error: null };
  try {
    if (mode === 'encode') {
      return { output: scope === 'component' ? encodeURIComponent(input) : encodeURI(input), error: null };
    }
    return { output: scope === 'component' ? decodeURIComponent(input) : decodeURI(input), error: null };
  } catch (e) {
    return { output: '', error: e instanceof Error ? e.message : 'Malformed URI sequence' };
  }
}