Use explicit comparisons instead of using Boolean constructor

src/error/GraphQLError.js

@@ -157,7 +157,7 @@ export function GraphQLError( // eslint-disable-line no-redeclare
       // By being enumerable, JSON.stringify will include `locations` in the
       // resulting output. This ensures that the simplest possible GraphQL
       // service adheres to the spec.
-      enumerable: Boolean(_locations),
+      enumerable: _locations != null,
     },
     path: {
       // Coercing falsey values to undefined ensures they will not be included
@@ -166,7 +166,7 @@ export function GraphQLError( // eslint-disable-line no-redeclare
       // By being enumerable, JSON.stringify will include `path` in the
       // resulting output. This ensures that the simplest possible GraphQL
       // service adheres to the spec.
-      enumerable: Boolean(path),
+      enumerable: path != null,
     },
     nodes: {
       value: _nodes || undefined,
@@ -187,7 +187,7 @@ export function GraphQLError( // eslint-disable-line no-redeclare
       // By being enumerable, JSON.stringify will include `path` in the
       // resulting output. This ensures that the simplest possible GraphQL
       // service adheres to the spec.
-      enumerable: Boolean(_extensions),
+      enumerable: _extensions != null,
     },
   });
 

src/jsutils/isPromise.js

@@ -9,5 +9,5 @@ declare function isPromise(value: mixed): boolean %checks(value instanceof
 
 // eslint-disable-next-line no-redeclare
 export default function isPromise(value) {
-  return Boolean(value && typeof value.then === 'function');
+  return value != null && typeof value.then === 'function';
 }

src/language/visitor.js

@@ -346,7 +346,7 @@ export function visit(
 }
 
 function isNode(maybeNode): boolean %checks {
-  return Boolean(maybeNode && typeof maybeNode.kind === 'string');
+  return maybeNode != null && typeof maybeNode.kind === 'string';
 }
 
 /**

src/type/schema.js

@@ -247,7 +247,7 @@ export class GraphQLSchema {
       this._possibleTypeMap[abstractType.name] = map;
     }
 
-    return Boolean(this._possibleTypeMap[abstractType.name][possibleType.name]);
+    return this._possibleTypeMap[abstractType.name][possibleType.name] != null;
   }
 
   getDirectives(): $ReadOnlyArray<GraphQLDirective> {

src/validation/rules/KnownTypeNames.js

@@ -48,7 +48,7 @@ export function KnownTypeNames(
       const typeName = node.name.value;
       if (!existingTypesMap[typeName] && !definedTypes[typeName]) {
         const definitionNode = ancestors[2] || parent;
-        const isSDL = isSDLNode(definitionNode);
+        const isSDL = definitionNode != null && isSDLNode(definitionNode);
         if (isSDL && isSpecifiedScalarName(typeName)) {
           return;
         }
@@ -73,10 +73,9 @@ function isSpecifiedScalarName(typeName) {
   return specifiedScalarsNames.indexOf(typeName) !== -1;
 }
 
-function isSDLNode(value: ASTNode | $ReadOnlyArray<ASTNode> | void): boolean {
-  return Boolean(
-    value &&
-      !Array.isArray(value) &&
-      (isTypeSystemDefinitionNode(value) || isTypeSystemExtensionNode(value)),
+function isSDLNode(value: ASTNode | $ReadOnlyArray<ASTNode>): boolean {
+  return (
+    !Array.isArray(value) &&
+    (isTypeSystemDefinitionNode(value) || isTypeSystemExtensionNode(value))
   );
 }