Optimize shouldInclude for common case of no directives.

Since shouldInclude gets called for every single field in any read or
write operation, it's important that it takes any available shortcuts to
handle the common case (no directives) as cheaply as possible.

src/utilities/graphql/directives.ts

@@ -19,15 +19,18 @@ export type DirectiveInfo = {
 };
 
 export function shouldInclude(
-  selection: SelectionNode,
-  variables: { [name: string]: any } = {},
+  { directives }: SelectionNode,
+  variables?: Record<string, any>,
 ): boolean {
+  if (!directives || !directives.length) {
+    return true;
+  }
   return getInclusionDirectives(
-    selection.directives,
+    directives
   ).every(({ directive, ifArgument }) => {
     let evaledValue: boolean = false;
     if (ifArgument.value.kind === 'Variable') {
-      evaledValue = variables[(ifArgument.value as VariableNode).name.value];
+      evaledValue = variables && variables[(ifArgument.value as VariableNode).name.value];
       invariant(
         evaledValue !== void 0,
         `Invalid variable referenced in @${directive.name.value} directive.`,
@@ -77,31 +80,39 @@ function isInclusionDirective({ name: { value } }: DirectiveNode): boolean {
 export function getInclusionDirectives(
   directives: ReadonlyArray<DirectiveNode>,
 ): InclusionDirectives {
-  return directives ? directives.filter(isInclusionDirective).map(directive => {
-    const directiveArguments = directive.arguments;
-    const directiveName = directive.name.value;
-
-    invariant(
-      directiveArguments && directiveArguments.length === 1,
-      `Incorrect number of arguments for the @${directiveName} directive.`,
-    );
-
-    const ifArgument = directiveArguments[0];
-    invariant(
-      ifArgument.name && ifArgument.name.value === 'if',
-      `Invalid argument for the @${directiveName} directive.`,
-    );
-
-    const ifValue: ValueNode = ifArgument.value;
-
-    // means it has to be a variable value if this is a valid @skip or @include directive
-    invariant(
-      ifValue &&
-        (ifValue.kind === 'Variable' || ifValue.kind === 'BooleanValue'),
-      `Argument for the @${directiveName} directive must be a variable or a boolean value.`,
-    );
-
-    return { directive, ifArgument };
-  }) : [];
+  const result: InclusionDirectives = [];
+
+  if (directives && directives.length) {
+    directives.forEach(directive => {
+      if (!isInclusionDirective(directive)) return;
+
+      const directiveArguments = directive.arguments;
+      const directiveName = directive.name.value;
+
+      invariant(
+        directiveArguments && directiveArguments.length === 1,
+        `Incorrect number of arguments for the @${directiveName} directive.`,
+      );
+
+      const ifArgument = directiveArguments[0];
+      invariant(
+        ifArgument.name && ifArgument.name.value === 'if',
+        `Invalid argument for the @${directiveName} directive.`,
+      );
+
+      const ifValue: ValueNode = ifArgument.value;
+
+      // means it has to be a variable value if this is a valid @skip or @include directive
+      invariant(
+        ifValue &&
+          (ifValue.kind === 'Variable' || ifValue.kind === 'BooleanValue'),
+        `Argument for the @${directiveName} directive must be a variable or a boolean value.`,
+      );
+
+      result.push({ directive, ifArgument });
+    });
+  }
+
+  return result;
 }