`var JSONbigString = require('json-bigint');

var key = '{ "key": 1234567890123456789 }';
var withString = JSONbigString.parse(key);

console.log(Function.prototype.toString.call(withString.constructor));`

Result
`console.log(Function.prototype.toString.call(withString.constructor));
^

TypeError: Function.prototype.toString requires that 'this' be a Function
at toString ()
at Object. (Ravi/office/mix/check.js:6:41)
at Module._compile (internal/modules/cjs/loader.js:776:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:787:10)
at Module.load (internal/modules/cjs/loader.js:653:32)
at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
at Function.Module._load (internal/modules/cjs/loader.js:585:3)
at Function.Module.runMain (internal/modules/cjs/loader.js:829:12)
at startup (internal/bootstrap/node.js:283:19)
at bootstrapNodeJSCore (internal/bootstrap/node.js:622:3)`

Expected

function Object() { [native code] }

When you use normal JSON.parse you will get the constructor as expected.