diff --git a/src/flatten-map.ts b/src/flatten-map.ts
index 2b8c558..61ac40c 100644
--- a/src/flatten-map.ts
+++ b/src/flatten-map.ts
@@ -6,6 +6,7 @@ import {
   SOURCE_COLUMN,
   NAMES_INDEX,
 } from './sourcemap-segment';
+import { parse } from './types';
 
 import type {
   DecodedSourceMap,
@@ -14,12 +15,13 @@ import type {
   SectionedSourceMapXInput,
   SectionedSourceMapInput,
   SectionXInput,
+  Ro,
 } from './types';
 import type { SourceMapSegment } from './sourcemap-segment';
 
 type FlattenMap = {
-  new (map: SectionedSourceMapInput, mapUrl?: string | null): TraceMap;
-  (map: SectionedSourceMapInput, mapUrl?: string | null): TraceMap;
+  new (map: Ro<SectionedSourceMapInput>, mapUrl?: string | null): TraceMap;
+  (map: Ro<SectionedSourceMapInput>, mapUrl?: string | null): TraceMap;
 };
 
 export const FlattenMap: FlattenMap = function (map, mapUrl) {
@@ -62,10 +64,6 @@ export const FlattenMap: FlattenMap = function (map, mapUrl) {
   return presortedDecodedMap(joined);
 } as FlattenMap;
 
-function parse<T>(map: T): Exclude<T, string> {
-  return typeof map === 'string' ? JSON.parse(map) : map;
-}
-
 function recurse(
   input: SectionedSourceMapXInput,
   mapUrl: string | null | undefined,
diff --git a/src/trace-mapping.ts b/src/trace-mapping.ts
index d9be093..a5e85bf 100644
--- a/src/trace-mapping.ts
+++ b/src/trace-mapping.ts
@@ -20,6 +20,7 @@ import {
   REV_GENERATED_LINE,
   REV_GENERATED_COLUMN,
 } from './sourcemap-segment';
+import { parse } from './types';
 
 import type { SourceMapSegment, ReverseSegment } from './sourcemap-segment';
 import type {
@@ -38,6 +39,7 @@ import type {
   Bias,
   XInput,
   SectionedSourceMap,
+  Ro,
 } from './types';
 import type { Source } from './by-source';
 import type { MemoState } from './binary-search';
@@ -82,7 +84,7 @@ const COL_GTR_EQ_ZERO = '`column` must be greater than or equal to 0 (columns st
 export const LEAST_UPPER_BOUND = -1;
 export const GREATEST_LOWER_BOUND = 1;
 
-export { FlattenMap, FlattenMap as AnyMap } from './any-map';
+export { FlattenMap, FlattenMap as AnyMap } from './flatten-map';
 
 export class TraceMap implements SourceMap {
   declare version: SourceMapV3['version'];
@@ -102,12 +104,11 @@ export class TraceMap implements SourceMap {
   private declare _bySources: Source[] | undefined;
   private declare _bySourceMemos: MemoState[] | undefined;
 
-  constructor(map: SourceMapInput, mapUrl?: string | null) {
+  constructor(map: Ro<SourceMapInput>, mapUrl?: string | null) {
     const isString = typeof map === 'string';
-
     if (!isString && (map as unknown as { _decodedMemo: any })._decodedMemo) return map as TraceMap;
 
-    const parsed = (isString ? JSON.parse(map) : map) as DecodedSourceMap | EncodedSourceMap;
+    const parsed = parse(map as Exclude<SourceMapInput, TraceMap>);
 
     const { version, file, names, sourceRoot, sources, sourcesContent } = parsed;
     this.version = version;
diff --git a/src/types.ts b/src/types.ts
index 6b54b24..96a4370 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -102,3 +102,15 @@ export abstract class SourceMap {
   declare resolvedSources: SourceMapV3['sources'];
   declare ignoreList: SourceMapV3['ignoreList'];
 }
+
+export type Ro<T> = T extends Array<infer V>
+  ? V[] | Readonly<V[]> | RoArray<V> | Readonly<RoArray<V>>
+  : T extends object
+    ? T | Readonly<T> | RoObject<T> | Readonly<RoObject<T>>
+    : T;
+type RoArray<T> = Ro<T>[];
+type RoObject<T> = { [K in keyof T]: T[K] | Ro<T[K]> };
+
+export function parse<T>(map: T): Exclude<T, string> {
+  return typeof map === 'string' ? JSON.parse(map) : map;
+}