Added constant JSON imports and importJsonAsConst

microsoft · james-pre · Jan 18, 2026 · Jan 18, 2026 · Jan 18, 2026 · Jan 18, 2026
commit c5b1deec5a1086618a9735cf35fe0ce9494794c4
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -12515,6 +12515,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (!declaration.statements.length) {
                 return emptyObjectType;
             }
+            if (compilerOptions.importJsonAsConst) {
+                return checkExpression(declaration.statements[0].expression);
+            }
             return getWidenedType(getWidenedLiteralType(checkExpression(declaration.statements[0].expression)));
         }
         if (isAccessor(declaration)) {
@@ -41395,6 +41398,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
     function isConstContext(node: Expression): boolean {
         const parent = node.parent;
+        if (compilerOptions.importJsonAsConst && parent.kind === SyntaxKind.ExpressionStatement && isSourceFile(parent.parent) && isJsonSourceFile(parent.parent)) {
+            return true;
+        }
         return isAssertionExpression(parent) && isConstTypeReference(parent.type) ||
             isJSDocTypeAssertion(parent) && isConstTypeReference(getJSDocTypeAssertionType(parent)) ||
             isValidConstAssertionArgument(node) && isConstTypeVariable(getContextualType(node, ContextFlags.None)) ||

diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
@@ -1343,6 +1343,15 @@ const commandOptionsWithoutBuild: CommandLineOption[] = [
         description: Diagnostics.Enable_importing_json_files,
         defaultValueDescription: false,
     },
+    {
+        name: "importJsonAsConst",
+        type: "boolean",
+        affectsSemanticDiagnostics: true,
+        affectsBuildInfo: true,
+        category: Diagnostics.Language_and_Environment,
+        description: Diagnostics.Import_JSON_files_as_const_assertions,
+        defaultValueDescription: false,
+    },
     {
         name: "allowArbitraryExtensions",
         type: "boolean",

diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
@@ -6719,6 +6719,10 @@
         "category": "Message",
         "code": 6932
     },
+    "Import JSON files as const assertions.": {
+        "category": "Message",
+        "code": 6933
+    },
 
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",

diff --git a/src/compiler/types.ts b/src/compiler/types.ts
@@ -7442,6 +7442,7 @@ export interface CompilerOptions {
     /** @internal */ help?: boolean;
     ignoreDeprecations?: string;
     importHelpers?: boolean;
+    importJsonAsConst?: boolean;
     /** @deprecated */
     importsNotUsedAsValues?: ImportsNotUsedAsValues;
     /** @internal */ init?: boolean;