Added toJSONObject util; (#5247)

Fixed AxiosError.toJSON method to avoid circular references;

Co-authored-by: Jay <jasonsaayman@gmail.com>

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
 };