fix: #3469 add ResultSet interface and improve isResultSet typing (#3481)

Co-authored-by: Jos de Jong <[email protected]>

Author: ranidam

test/typescript-tests/testTypes.ts

@@ -48,6 +48,8 @@ import {
   SLUDecomposition,
   SymbolNode,
   Unit,
+  evaluate,
+  isResultSet,
   UnitPrefix
 } from 'mathjs'
 
@@ -171,6 +173,12 @@ Bignumbers examples
     )
   }
 }
+const result = evaluate('1 + 1; 2 + 2;') // multi-expression input
+
+if (isResultSet(result)) {
+  const entries = result.valueOf() // ✅ should be typed as unknown[]
+  const _last = entries.slice(-1)[0] // ✅ access last result safely
+}
 
 /*
   Arithmetic function examples

types/index.d.ts

@@ -3518,7 +3518,7 @@ export interface MathJsInstance extends MathJsFactory {
 
   isBoolean(x: unknown): x is boolean
 
-  isResultSet(x: unknown): boolean
+  isResultSet(x: unknown): x is ResultSet
 
   isHelp(x: unknown): x is Help
 
@@ -4319,6 +4319,14 @@ export interface EvalFunction {
   evaluate(scope?: any): any
 }
 
+// ResultSet type and helper
+export interface ResultSet {
+  entries: unknown[]
+  valueOf(): unknown[]
+  toString(): string
+  toJSON(): MathJSON
+}
+
 export interface MathNode {
   isNode: true
   comment: string