flow like the river

VISUALIZACION/node_modules/graphql/validation/rules/VariablesInAllowedPositionRule.js.flow

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+// @flow strict
+import inspect from '../../jsutils/inspect';
+
+import { GraphQLError } from '../../error/GraphQLError';
+
+import { Kind } from '../../language/kinds';
+import type { ValueNode } from '../../language/ast';
+import type { ASTVisitor } from '../../language/visitor';
+
+import type { GraphQLSchema } from '../../type/schema';
+import type { GraphQLType } from '../../type/definition';
+import { isNonNullType } from '../../type/definition';
+
+import { typeFromAST } from '../../utilities/typeFromAST';
+import { isTypeSubTypeOf } from '../../utilities/typeComparators';
+
+import type { ValidationContext } from '../ValidationContext';
+
+/**
+ * Variables passed to field arguments conform to type
+ */
+export function VariablesInAllowedPositionRule(
+  context: ValidationContext,
+): ASTVisitor {
+  let varDefMap = Object.create(null);
+
+  return {
+    OperationDefinition: {
+      enter() {
+        varDefMap = Object.create(null);
+      },
+      leave(operation) {
+        const usages = context.getRecursiveVariableUsages(operation);
+
+        for (const { node, type, defaultValue } of usages) {
+          const varName = node.name.value;
+          const varDef = varDefMap[varName];
+          if (varDef && type) {
+            // A var type is allowed if it is the same or more strict (e.g. is
+            // a subtype of) than the expected type. It can be more strict if
+            // the variable type is non-null when the expected type is nullable.
+            // If both are list types, the variable item type can be more strict
+            // than the expected item type (contravariant).
+            const schema = context.getSchema();
+            const varType = typeFromAST(schema, varDef.type);
+            if (
+              varType &&
+              !allowedVariableUsage(
+                schema,
+                varType,
+                varDef.defaultValue,
+                type,
+                defaultValue,
+              )
+            ) {
+              const varTypeStr = inspect(varType);
+              const typeStr = inspect(type);
+              context.reportError(
+                new GraphQLError(
+                  `Variable "$${varName}" of type "${varTypeStr}" used in position expecting type "${typeStr}".`,
+                  [varDef, node],
+                ),
+              );
+            }
+          }
+        }
+      },
+    },
+    VariableDefinition(node) {
+      varDefMap[node.variable.name.value] = node;
+    },
+  };
+}
+
+/**
+ * Returns true if the variable is allowed in the location it was found,
+ * which includes considering if default values exist for either the variable
+ * or the location at which it is located.
+ */
+function allowedVariableUsage(
+  schema: GraphQLSchema,
+  varType: GraphQLType,
+  varDefaultValue: ?ValueNode,
+  locationType: GraphQLType,
+  locationDefaultValue: ?mixed,
+): boolean {
+  if (isNonNullType(locationType) && !isNonNullType(varType)) {
+    const hasNonNullVariableDefaultValue =
+      varDefaultValue != null && varDefaultValue.kind !== Kind.NULL;
+    const hasLocationDefaultValue = locationDefaultValue !== undefined;
+    if (!hasNonNullVariableDefaultValue && !hasLocationDefaultValue) {
+      return false;
+    }
+    const nullableLocationType = locationType.ofType;
+    return isTypeSubTypeOf(schema, varType, nullableLocationType);
+  }
+  return isTypeSubTypeOf(schema, varType, locationType);
+}