refactor: Removed dependency on lodash.assign (optimizely#395)

Summary:

to reduce package size, we are trying to gradually get rid of lodash library. This PR replaces assign function with Object.assign. Also added a polyfill method to support old browsers. Polyfill was taken from MDN. 

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign#Polyfill

Test Plan:

All tests pass after this change

Author: zashraf1985

packages/optimizely-sdk/lib/core/audience_evaluator/index.js

@@ -35,7 +35,7 @@ var MODULE_NAME = 'AUDIENCE_EVALUATOR';
  * @constructor
  */
 function AudienceEvaluator(UNSTABLE_conditionEvaluators) {
-  this.typeToEvaluatorMap = fns.assignIn({}, UNSTABLE_conditionEvaluators, {
+  this.typeToEvaluatorMap = fns.assign({}, UNSTABLE_conditionEvaluators, {
     'custom_attribute': customAttributeConditionEvaluator
   });
 }

packages/optimizely-sdk/lib/core/decision_service/index.js

@@ -116,7 +116,7 @@ DecisionService.prototype.__resolveExperimentBucketMap = function(userId, attrib
   attributes = attributes || {}
   var userProfile = this.__getUserProfile(userId) || {};
   var attributeExperimentBucketMap = attributes[enums.CONTROL_ATTRIBUTES.STICKY_BUCKETING_KEY];
-  return fns.assignIn({}, userProfile.experiment_bucket_map, attributeExperimentBucketMap);
+  return fns.assign({}, userProfile.experiment_bucket_map, attributeExperimentBucketMap);
 };
 
 

packages/optimizely-sdk/lib/core/project_config/index.js

@@ -55,7 +55,7 @@ module.exports = {
     fns.forEach(projectConfig.groupIdMap, function(group, Id) {
       experiments = fns.cloneDeep(group.experiments);
       fns.forEach(experiments, function(experiment) {
-        projectConfig.experiments.push(fns.assignIn(experiment, {groupId: Id}));
+        projectConfig.experiments.push(fns.assign(experiment, {groupId: Id}));
       });
     });
 
@@ -78,7 +78,7 @@ module.exports = {
       experiment.variationKeyMap = fns.keyBy(experiment.variations, 'key');
 
       // Creates { <variationId>: { key: <variationKey>, id: <variationId> } } mapping for quick lookup
-      fns.assignIn(projectConfig.variationIdMap, fns.keyBy(experiment.variations, 'id'));
+      fns.assign(projectConfig.variationIdMap, fns.keyBy(experiment.variations, 'id'));
 
       fns.forOwn(experiment.variationKeyMap, function(variation) {
         if (variation.variables) {

packages/optimizely-sdk/lib/index.browser.js

@@ -107,7 +107,7 @@ module.exports = {
         eventDispatcher = config.eventDispatcher;
       }
 
-      config = fns.assignIn(
+      config = fns.assign(
         {
           clientEngine: enums.JAVASCRIPT_CLIENT_ENGINE,
           eventBatchSize: DEFAULT_EVENT_BATCH_SIZE,

packages/optimizely-sdk/lib/index.react_native.js

@@ -86,7 +86,7 @@ module.exports = {
         config.skipJSONValidation = true;
       }
 
-      config = fns.assignIn(
+      config = fns.assign(
         {
           clientEngine: enums.JAVASCRIPT_CLIENT_ENGINE,
           eventBatchSize: DEFAULT_EVENT_BATCH_SIZE,

packages/optimizely-sdk/lib/utils/fns/index.js

@@ -19,8 +19,28 @@ var keyBy = require('@optimizely/js-sdk-utils').keyBy;
 var MAX_NUMBER_LIMIT = Math.pow(2, 53);
 
 module.exports = {
-  assign: require('lodash/assign'),
-  assignIn: require('lodash/assignIn'),
+  assign: function (target) {
+    if (!target) {
+      return {};
+    }
+    if (typeof Object.assign === 'function') {
+      return Object.assign.apply(Object, arguments);
+    } else {
+      var to = Object(target);
+      for (var index = 1; index < arguments.length; index++) {
+        var nextSource = arguments[index];
+        if (nextSource !== null && nextSource !== undefined) { 
+          for (var nextKey in nextSource) {
+            // Avoid bugs when hasOwnProperty is shadowed
+            if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
+              to[nextKey] = nextSource[nextKey];
+            }
+          }
+        }
+      }
+      return to;
+    }
+  },
   cloneDeep: require('lodash/cloneDeep'),
   currentTimestamp: function() {
     return Math.round(new Date().getTime());