Fixed AxiosError.toJSON method to avoid circular references;

lib/core/AxiosError.js

@@ -45,7 +45,7 @@ utils.inherits(AxiosError, Error, {
       columnNumber: this.columnNumber,
       stack: this.stack,
       // Axios
-      config: this.config,
+      config: utils.toJSONObject(this.config),
       code: this.code,
       status: this.response && this.response.status ? this.response.status : null
     };

lib/utils.js

@@ -592,6 +592,37 @@ const toFiniteNumber = (value, defaultValue) => {
   return Number.isFinite(value) ? value : defaultValue;
 }
 
+const toJSONObject = (obj) => {
+  const stack = new Array(10);
+
+  const visit = (source, i) => {
+
+    if (isObject(source)) {
+      if (stack.indexOf(source) >= 0) {
+        return;
+      }
+
+      if(!('toJSON' in source)) {
+        stack[i] = source;
+        const target = isArray(source) ? [] : {};
+
+        forEach(source, (value, key) => {
+          const reducedValue = visit(value, i + 1);
+          !isUndefined(reducedValue) && (target[key] = reducedValue);
+        });
+
+        stack[i] = undefined;
+
+        return target;
+      }
+    }
+
+    return source;
+  }
+
+  return visit(obj, 0);
+}
+
 export default {
   isArray,
   isArrayBuffer,
@@ -637,5 +668,6 @@ export default {
   toFiniteNumber,
   findKey,
   global: _global,
-  isContextDefined
+  isContextDefined,
+  toJSONObject
 };

test/unit/utils/utils.js

@@ -23,4 +23,30 @@ describe('utils', function (){
       assert.equal(utils.isFormData(new FormData()), true);
     });
   });
+
+  describe('toJSON', function (){
+    it('should convert to a plain object without circular references', function () {
+      const obj= {a: [0]}
+      const source = {x:1, y:2, obj};
+      source.circular1 = source;
+      obj.a[1] = obj;
+
+      assert.deepStrictEqual(utils.toJSONObject(source), {
+        x: 1, y:2, obj: {a: [0]}
+      });
+    });
+
+    it('should use objects with defined toJSON method without rebuilding', function () {
+      const objProp = {};
+      const obj= {objProp, toJSON(){
+        return {ok: 1}
+      }};
+      const source = {x:1, y:2, obj};
+
+      const jsonObject = utils.toJSONObject(source);
+
+      assert.strictEqual(jsonObject.obj.objProp, objProp);
+      assert.strictEqual(JSON.stringify(jsonObject), JSON.stringify({x: 1, y:2, obj: {ok: 1}}))
+    });
+  });
 });