在JSON.stringify（）中，instanceof的行为有何不同？

Question

I'm using decimal.js for some financial calculations in Node. I'm writing a custom JSON.stringify replacer function , but when I test the property types using instanceof , I get a different result than when I do the same test outside of the replacer function.

Here's a runnable example:

 const myObj = { myNum: new Decimal(0.3) }; // logs 'Property "myNum" is a Decimal: true' console.log('Property "myNum" is a Decimal:', myObj.myNum instanceof Decimal); const replacer = (key, value) => { if (key === 'myNum') { // logs 'Property "myNum" is a Decimal: false' console.log('Property "myNum" is a Decimal:', value instanceof Decimal); } if (value instanceof Decimal) { return value.toNumber() } else { return value; } } JSON.stringify(myObj, replacer, 4); 

 <script src="https://cdnjs.cloudflare.com/ajax/libs/decimal.js/10.0.0/decimal.js"></script> 

Why is this happening?

If I replace the Decimal instance with an instance of my own custom class, both instanceof tests behave the same, as expected:

 function MyClass() {} const myObj = { myClass: new MyClass() }; // logs 'Property "myClass" is a MyClass: true' console.log('Property "myClass" is a MyClass:', myObj.myClass instanceof MyClass); const replacer = (key, value) => { if (key === 'myClass') { // logs 'Property "myClass" is a MyClass: true' console.log('Property "myClass" is a MyClass:', value instanceof MyClass); } return value; } JSON.stringify(myObj, replacer, 4); 

Answer 1

Figured it out. The Decimal instance includes a .toJSON() method. When JSON.stringify encounters an object that defines a toJSON function, it calls it and returns the result as the second parameter in the replacer function instead of the object reference. As a result, the value variable in my example above pointed to a string , not a Decimal instance.

From MDN:

If an object being stringified has a property named toJSON whose value is a function, then the toJSON() method customizes JSON stringification behavior: instead of the object being serialized, the value returned by the toJSON() method when called will be serialized.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#toJSON()_behavior

To demonstrate this, I can tweak my second example above to include a toJSON function:

 function MyClass() { // add a toJSON method to my custom class this.toJSON = () => { return 'Hello, world!'; }; }; const myObj = { myClass: new MyClass() }; // logs 'Property "myClass" is a MyClass: true' console.log('Property "myClass" is a MyClass:', myObj.myClass instanceof MyClass); const replacer = (key, value) => { if (key === 'myClass') { // logs 'Property "myClass" is a MyClass: true' console.log('Property "myClass" is a MyClass:', value instanceof MyClass); } return value; } JSON.stringify(myObj, replacer, 4);