fix the return type of stringify

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased](https://github.com/hyparam/hightable/compare/v0.13.1...HEAD)
 
+### Refactor
+
+- fix the return type of `stringify` ([#85](https://github.com/hyparam/hightable/pull/85)).
+
 ## [0.13.1](https://github.com/hyparam/hightable/compare/v0.13.0...v0.13.1) - 2025-03-27
 
 ### Changed

src/utils/stringify.ts

@@ -1,13 +1,15 @@
 /**
  * Robust stringification of any value, including json and bigints.
  */
-export function stringify(value: unknown): string {
+export function stringify(value: unknown): string | undefined {
   if (typeof value === 'string') return value
   if (typeof value === 'number' || typeof value === 'bigint') return value.toLocaleString('en-US')
   if (typeof value === 'boolean') return value.toString()
   if (Array.isArray(value)) {
     return `[\n${value.map((v) => indent(stringify(v), 2)).join(',\n')}\n]`
   }
+  // Beware: JSON.stringify is not robustly typed since it returns undefined (not a string) for undefined:
+  // More details and corner cases here: https://github.com/microsoft/TypeScript/issues/18879#issuecomment-1399758565
   if (value === null || value === undefined) return JSON.stringify(value)
   if (value instanceof Date) return value.toISOString()
   if (typeof value === 'object') {
@@ -16,7 +18,8 @@ export function stringify(value: unknown): string {
       .map(([k, v]) => `${k}: ${stringify(v)}`)
       .join(', ')}}`
   }
-  return '{}'
+  // Fallback to JSON.stringify for unknown types or corner cases (e.g. functions)
+  return JSON.stringify(value)
 }
 
 function indent(text: string | undefined, spaces: number) {