Convert more cycles to 'for..of' (#2100)

Since #2099 makes it zero-cost to use 'for..of' it's better to improve
code readability and consistency

src/execution/execute.js

@@ -281,8 +281,7 @@ export function buildExecutionContext(
   let operation: OperationDefinitionNode | void;
   let hasMultipleAssumedOperations = false;
   const fragments: ObjMap<FragmentDefinitionNode> = Object.create(null);
-  for (let i = 0; i < document.definitions.length; i++) {
-    const definition = document.definitions[i];
+  for (const definition of document.definitions) {
     switch (definition.kind) {
       case Kind.OPERATION_DEFINITION:
         if (!operationName && operation) {
@@ -434,8 +433,7 @@ function executeFields(
   const results = Object.create(null);
   let containsPromise = false;
 
-  for (let i = 0, keys = Object.keys(fields); i < keys.length; ++i) {
-    const responseName = keys[i];
+  for (const responseName of Object.keys(fields)) {
     const fieldNodes = fields[responseName];
     const fieldPath = addPath(path, responseName);
     const result = resolveField(
@@ -480,8 +478,7 @@ export function collectFields(
   fields: ObjMap<Array<FieldNode>>,
   visitedFragmentNames: ObjMap<boolean>,
 ): ObjMap<Array<FieldNode>> {
-  for (let i = 0; i < selectionSet.selections.length; i++) {
-    const selection = selectionSet.selections[i];
+  for (const selection of selectionSet.selections) {
     switch (selection.kind) {
       case Kind.FIELD: {
         if (!shouldIncludeNode(exeContext, selection)) {
@@ -1108,13 +1105,12 @@ function _collectSubfields(
 ): ObjMap<Array<FieldNode>> {
   let subFieldNodes = Object.create(null);
   const visitedFragmentNames = Object.create(null);
-  for (let i = 0; i < fieldNodes.length; i++) {
-    const selectionSet = fieldNodes[i].selectionSet;
-    if (selectionSet) {
+  for (const node of fieldNodes) {
+    if (node.selectionSet) {
       subFieldNodes = collectFields(
         exeContext,
         returnType,
-        selectionSet,
+        node.selectionSet,
         subFieldNodes,
         visitedFragmentNames,
       );

src/jsutils/suggestionList.js

@@ -45,8 +45,6 @@ function lexicalDistance(aStr, bStr) {
     return 0;
   }
 
-  let i;
-  let j;
   const d = [];
   const a = aStr.toLowerCase();
   const b = bStr.toLowerCase();
@@ -58,16 +56,16 @@ function lexicalDistance(aStr, bStr) {
     return 1;
   }
 
-  for (i = 0; i <= aLength; i++) {
+  for (let i = 0; i <= aLength; i++) {
     d[i] = [i];
   }
 
-  for (j = 1; j <= bLength; j++) {
+  for (let j = 1; j <= bLength; j++) {
     d[0][j] = j;
   }
 
-  for (i = 1; i <= aLength; i++) {
-    for (j = 1; j <= bLength; j++) {
+  for (let i = 1; i <= aLength; i++) {
+    for (let j = 1; j <= bLength; j++) {
       const cost = a[i - 1] === b[j - 1] ? 0 : 1;
 
       d[i][j] = Math.min(

src/polyfills/find.js

@@ -12,8 +12,7 @@ const find = Array.prototype.find
       return Array.prototype.find.call(list, predicate);
     }
   : function(list, predicate) {
-      for (let i = 0; i < list.length; i++) {
-        const value = list[i];
+      for (const value of list) {
         if (predicate(value)) {
           return value;
         }

src/polyfills/flatMap.js

@@ -13,8 +13,8 @@ const flatMap = Array.prototype.flatMap
     }
   : function(list, fn) {
       let result = [];
-      for (let i = 0; i < list.length; i++) {
-        const value = fn(list[i]);
+      for (const item of list) {
+        const value = fn(item);
         if (Array.isArray(value)) {
           result = result.concat(value);
         } else {

src/validation/rules/OverlappingFieldsCanBeMerged.js

@@ -747,8 +747,7 @@ function _collectFieldsAndFragmentNames(
   nodeAndDefs,
   fragmentNames,
 ): void {
-  for (let i = 0; i < selectionSet.selections.length; i++) {
-    const selection = selectionSet.selections[i];
+  for (const selection of selectionSet.selections) {
     switch (selection.kind) {
       case Kind.FIELD: {
         const fieldName = selection.name.value;