JSON.stringify-replacer.js

// Replacer function meant for `JSON.stringify()`.
function getCircularReplacer() {
  const seen = new WeakSet();
  return (_key, value) => {
    // Non-null "object" properties: return a deep copy without circular references.
    if (typeof value === "object" && value !== null) {
      // Remove circular references.
      if (seen.has(value)) {
        // Option 1: Replace circular references with a string.
        return "(circular)";

        // Option 2: Return `undefined` to just skip circular references.
        // return;
      }
      seen.add(value);

      // Make a deep copy where all properties are enumerable.
      const copy = Array.isArray(value) ? [] : {};
      Object.getOwnPropertyNames(value).forEach((propName) => {
        copy[propName] = value[propName];
      });
      return copy;
    }

    // Rest of properties: return as-is.
    return value;
  };
}

// The Error type has some non-enumerable properties: "stack", "message".
const myError = new Error("My Error message");
myError["a"] = "My property";

console.log(myError);
// Calls `myError.toString()` which is not a good serialized form:
// > Error: My Error message

console.log(JSON.stringify(myError));
// Doesn't print the non-enumerable properties:
// > {"a":"My property"}

myError.c = myError; // .c is a circular reference.

console.log(JSON.stringify(myError));
// Fails with:
// > TypeError: Converting circular structure to JSON
// >   --> starting at object with constructor 'Error'
// >   --- property 'c' closes the circle
// >   at JSON.stringify

console.log(JSON.stringify(myError, getCircularReplacer()));
// Removes circular references and prints the non-enumerable properties:
// > {"stack":"Error: My Error message\n    at <anonymous>:1:17","message":"My Error message","a":"My property","c":"(circular)"}