Allow for serializing int as BigInt

Author: JoviDeCroock

src/type/__tests__/scalars-test.ts

@@ -614,6 +614,7 @@ describe('Type System: Specified scalar types', () => {
       expect(serialize(123)).to.equal('123');
       expect(serialize(0)).to.equal('0');
       expect(serialize(-1)).to.equal('-1');
+      expect(serialize(BigInt(123))).to.equal('123');
 
       const valueOf = () => 'valueOf ID';
       const toJSON = () => 'toJSON ID';

src/type/scalars.ts

@@ -252,7 +252,7 @@ export const GraphQLID = new GraphQLScalarType<string>({
     if (typeof coercedValue === 'string') {
       return coercedValue;
     }
-    if (Number.isInteger(coercedValue)) {
+    if (Number.isInteger(coercedValue) || typeof coercedValue === 'bigint') {
       return String(coercedValue);
     }
     throw new GraphQLError(