[Robustness] stringify: avoid relying on a global undefined

Author: Connormiha

lib/stringify.js

@@ -80,7 +80,7 @@ var stringify = function stringify(
     var tmpSc = sideChannel;
     var step = 0;
     var findFlag = false;
-    while ((tmpSc = tmpSc.get(sentinel)) !== undefined && !findFlag) {
+    while ((tmpSc = tmpSc.get(sentinel)) !== void undefined && !findFlag) {
         // Where object last appeared in the ref tree
         var pos = tmpSc.get(object);
         step += 1;
@@ -142,7 +142,7 @@ var stringify = function stringify(
     var objKeys;
     if (generateArrayPrefix === 'comma' && isArray(obj)) {
         // we need to join elements in
-        objKeys = [{ value: obj.length > 0 ? obj.join(',') || null : undefined }];
+        objKeys = [{ value: obj.length > 0 ? obj.join(',') || null : void undefined }];
     } else if (isArray(filter)) {
         objKeys = filter;
     } else {
@@ -152,7 +152,7 @@ var stringify = function stringify(
 
     for (var j = 0; j < objKeys.length; ++j) {
         var key = objKeys[j];
-        var value = typeof key === 'object' && key.value !== undefined ? key.value : obj[key];
+        var value = typeof key === 'object' && typeof key.value !== 'undefined' ? key.value : obj[key];
 
         if (skipNulls && value === null) {
             continue;
@@ -192,7 +192,7 @@ var normalizeStringifyOptions = function normalizeStringifyOptions(opts) {
         return defaults;
     }
 
-    if (opts.encoder !== null && opts.encoder !== undefined && typeof opts.encoder !== 'function') {
+    if (opts.encoder !== null && typeof opts.encoder !== 'undefined' && typeof opts.encoder !== 'function') {
         throw new TypeError('Encoder has to be a function.');
     }