diff --git a/implementations/javascript/packages/core/src/annotated.ts b/implementations/javascript/packages/core/src/annotated.ts
index 054f570..d97bcf8 100644
--- a/implementations/javascript/packages/core/src/annotated.ts
+++ b/implementations/javascript/packages/core/src/annotated.ts
@@ -1,10 +1,10 @@
 import { Encoder } from "./encoder";
 import { Tag } from "./constants";
-import { AsPreserve, PreserveOn } from "./symbols";
 import { Value } from "./values";
 import { is, isAnnotated, IsPreservesAnnotated } from "./is";
 import { stringify } from "./text";
 import { GenericEmbedded } from "./embedded";
+import type { Preservable } from "./encoder";
 
 export interface Position {
     line?: number;
@@ -53,7 +53,7 @@ export function formatPosition(p: Position | null | string): string {
     }
 }
 
-export class Annotated<T = GenericEmbedded> {
+export class Annotated<T = GenericEmbedded> implements Preservable<T> {
     readonly annotations: Array<Value<T>>;
     readonly pos: Position | null;
     readonly item: Value<T>;
@@ -64,11 +64,15 @@ export class Annotated<T = GenericEmbedded> {
         this.item = item;
     }
 
-    [AsPreserve](): Value<T> {
+    __as__preserve__(): Value<T> {
         return this;
     }
 
-    [PreserveOn](encoder: Encoder<T>) {
+    static __from_preserve__<T>(v: Value<T>): undefined | Annotated<T> {
+        return isAnnotated<T>(v) ? v : void 0;
+    }
+
+    __preserve_on__(encoder: Encoder<T>): void {
         if (encoder.includeAnnotations) {
             for (const a of this.annotations) {
                 encoder.state.emitbyte(Tag.Annotation);
diff --git a/implementations/javascript/packages/core/src/bytes.ts b/implementations/javascript/packages/core/src/bytes.ts
index 6acafba..b7769e0 100644
--- a/implementations/javascript/packages/core/src/bytes.ts
+++ b/implementations/javascript/packages/core/src/bytes.ts
@@ -1,5 +1,4 @@
 import { Tag } from './constants';
-import { AsPreserve, PreserveOn } from './symbols';
 import { Encoder, Preservable } from './encoder';
 import { Value } from './values';
 import { GenericEmbedded } from './embedded';
@@ -127,10 +126,14 @@ export class Bytes implements Preservable<never> {
         return this.asPreservesText();
     }
 
-    [AsPreserve]<T = GenericEmbedded>(): Value<T> {
+    __as_preserve__<T = GenericEmbedded>(): Value<T> {
         return this;
     }
 
+    static __from_preserve__<T>(v: Value<T>): undefined | Bytes {
+        return Bytes.isBytes(v) ? v : void 0;
+    }
+
     asPreservesText(): string {
         return '#"' + this.__asciify() + '"';
     }
@@ -165,7 +168,7 @@ export class Bytes implements Preservable<never> {
         return this.toHex();
     }
 
-    [PreserveOn](encoder: Encoder<never>) {
+    __preserve_on__(encoder: Encoder<never>) {
         encoder.state.emitbyte(Tag.ByteString);
         encoder.state.varint(this.length);
         encoder.state.emitbytes(this._view);
diff --git a/implementations/javascript/packages/core/src/dictionary.ts b/implementations/javascript/packages/core/src/dictionary.ts
index de89d46..ccc4e66 100644
--- a/implementations/javascript/packages/core/src/dictionary.ts
+++ b/implementations/javascript/packages/core/src/dictionary.ts
@@ -1,16 +1,16 @@
 import { Encoder, canonicalEncode, canonicalString } from "./encoder";
 import { Tag } from "./constants";
 import { FlexMap, FlexSet, _iterMap } from "./flex";
-import { PreserveOn } from "./symbols";
 import { stringify } from "./text";
 import { Value } from "./values";
 import { Bytes } from './bytes';
 import { GenericEmbedded } from "./embedded";
+import type { Preservable } from "./encoder";
 
 export type DictionaryType = 'Dictionary' | 'Set';
 export const DictionaryType = Symbol.for('DictionaryType');
 
-export class KeyedDictionary<K extends Value<T>, V, T = GenericEmbedded> extends FlexMap<K, V> {
+export class KeyedDictionary<K extends Value<T>, V, T = GenericEmbedded> extends FlexMap<K, V> implements Preservable<T> {
     get [DictionaryType](): DictionaryType {
         return 'Dictionary';
     }
@@ -51,7 +51,7 @@ export class KeyedDictionary<K extends Value<T>, V, T = GenericEmbedded> extends
 
     get [Symbol.toStringTag]() { return 'Dictionary'; }
 
-    [PreserveOn](encoder: Encoder<T>) {
+    __preserve_on__(encoder: Encoder<T>) {
         if (encoder.canonical) {
             const entries = Array.from(this);
             const pieces = entries.map<[Bytes, number]>(([k, _v], i) => [canonicalEncode(k), i]);
@@ -78,9 +78,13 @@ export class Dictionary<T = GenericEmbedded, V = Value<T>> extends KeyedDictiona
     static isDictionary<T = GenericEmbedded, V = Value<T>>(x: any): x is Dictionary<T, V> {
         return x?.[DictionaryType] === 'Dictionary';
     }
+
+    static __from_preserve__<T>(v: Value<T>): undefined | Dictionary<T> {
+        return Dictionary.isDictionary<T>(v) ? v : void 0;
+    }
 }
 
-export class KeyedSet<K extends Value<T>, T = GenericEmbedded> extends FlexSet<K> {
+export class KeyedSet<K extends Value<T>, T = GenericEmbedded> extends FlexSet<K> implements Preservable<T> {
     get [DictionaryType](): DictionaryType {
         return 'Set';
     }
@@ -119,7 +123,7 @@ export class KeyedSet<K extends Value<T>, T = GenericEmbedded> extends FlexSet<K
 
     get [Symbol.toStringTag]() { return 'Set'; }
 
-    [PreserveOn](encoder: Encoder<T>) {
+    __preserve_on__(encoder: Encoder<T>) {
         if (encoder.canonical) {
             const pieces = Array.from(this).map<[Bytes, K]>(k => [canonicalEncode(k), k]);
             pieces.sort((a, b) => Bytes.compare(a[0], b[0]));
@@ -134,4 +138,8 @@ export class Set<T = GenericEmbedded> extends KeyedSet<Value<T>, T> {
     static isSet<T = GenericEmbedded>(x: any): x is Set<T> {
         return x?.[DictionaryType] === 'Set';
     }
+
+    static __from_preserve__<T>(v: Value<T>): undefined | Set<T> {
+        return Set.isSet<T>(v) ? v : void 0;
+    }
 }
diff --git a/implementations/javascript/packages/core/src/embedded.ts b/implementations/javascript/packages/core/src/embedded.ts
index e0bf7c2..bfbebdf 100644
--- a/implementations/javascript/packages/core/src/embedded.ts
+++ b/implementations/javascript/packages/core/src/embedded.ts
@@ -33,6 +33,14 @@ export class Embedded<T> {
             return '#!' + (this.embeddedValue as any).toString();
         }
     }
+
+    __as_preserve__<R>(): T extends R ? Value<R> : never {
+        return this as any;
+    }
+
+    static __from_preserve__<T>(v: Value<T>): undefined | Embedded<T> {
+        return isEmbedded<T>(v) ? v : void 0;
+    }
 }
 
 export function embed<T>(embeddedValue: T): Embedded<T> {
diff --git a/implementations/javascript/packages/core/src/encoder.ts b/implementations/javascript/packages/core/src/encoder.ts
index 0e222bd..fc0ccf6 100644
--- a/implementations/javascript/packages/core/src/encoder.ts
+++ b/implementations/javascript/packages/core/src/encoder.ts
@@ -1,7 +1,6 @@
 import { Tag } from "./constants";
 import { Bytes } from "./bytes";
 import { Value } from "./values";
-import { PreserveOn } from "./symbols";
 import { EncodeError } from "./codec";
 import { Record, Tuple } from "./record";
 import { GenericEmbedded, EmbeddedTypeEncode } from "./embedded";
@@ -10,11 +9,11 @@ export type Encodable<T> =
     Value<T> | Preservable<T> | Iterable<Value<T>> | ArrayBufferView;
 
 export interface Preservable<T> {
-    [PreserveOn](encoder: Encoder<T>): void;
+    __preserve_on__(encoder: Encoder<T>): void;
 }
 
 export function isPreservable<T>(v: any): v is Preservable<T> {
-    return typeof v === 'object' && v !== null && typeof v[PreserveOn] === 'function';
+    return typeof v === 'object' && v !== null && '__preserve_on__' in v && typeof v.__preserve_on__ === 'function';
 }
 
 export interface EncoderOptions {
@@ -211,11 +210,11 @@ export class Encoder<T = object> {
     }
 
     push(v: Encodable<T>) {
-        if (isPreservable<never>(v)) {
-            v[PreserveOn](this as unknown as Encoder<never>);
+        if (isPreservable<unknown>(v)) {
+            v.__preserve_on__(this);
         }
         else if (isPreservable<T>(v)) {
-            v[PreserveOn](this);
+            v.__preserve_on__(this);
         }
         else if (typeof v === 'boolean') {
             this.state.emitbyte(v ? Tag.True : Tag.False);
diff --git a/implementations/javascript/packages/core/src/float.ts b/implementations/javascript/packages/core/src/float.ts
index a2f0980..1b8c8c3 100644
--- a/implementations/javascript/packages/core/src/float.ts
+++ b/implementations/javascript/packages/core/src/float.ts
@@ -1,6 +1,5 @@
 import { Encoder, Preservable } from "./encoder";
 import { Tag } from "./constants";
-import { AsPreserve, PreserveOn } from "./symbols";
 import { Value } from "./values";
 import { GenericEmbedded } from "./embedded";
 
@@ -44,12 +43,16 @@ export function floatValue(f: any): number {
     }
 }
 
-export class SingleFloat extends Float implements Preservable<never> {
-    [AsPreserve]<T = GenericEmbedded>(): Value<T> {
+export class SingleFloat extends Float implements Preservable<any> {
+    __as_preserve__<T = GenericEmbedded>(): Value<T> {
         return this;
     }
 
-    [PreserveOn](encoder: Encoder<never>) {
+    static __from_preserve__<T>(v: Value<T>): undefined | SingleFloat {
+        return Float.isSingle(v) ? v : void 0;
+    }
+
+    __preserve_on__(encoder: Encoder<any>) {
         encoder.state.emitbyte(Tag.Float);
         encoder.state.makeroom(4);
         encoder.state.view.setFloat32(encoder.state.index, this.value, false);
@@ -69,12 +72,16 @@ export function Single(value: number | Float): SingleFloat {
     return new SingleFloat(value);
 }
 
-export class DoubleFloat extends Float implements Preservable<never> {
-    [AsPreserve]<T = GenericEmbedded>(): Value<T> {
+export class DoubleFloat extends Float implements Preservable<any> {
+    __as_preserve__<T = GenericEmbedded>(): Value<T> {
         return this;
     }
 
-    [PreserveOn](encoder: Encoder<never>) {
+    static __from_preserve__<T>(v: Value<T>): undefined | DoubleFloat {
+        return Float.isDouble(v) ? v : void 0;
+    }
+
+    __preserve_on__(encoder: Encoder<any>) {
         encoder.state.emitbyte(Tag.Double);
         encoder.state.makeroom(8);
         encoder.state.view.setFloat64(encoder.state.index, this.value, false);
diff --git a/implementations/javascript/packages/core/src/fromjs.ts b/implementations/javascript/packages/core/src/fromjs.ts
index 587d846..c676152 100644
--- a/implementations/javascript/packages/core/src/fromjs.ts
+++ b/implementations/javascript/packages/core/src/fromjs.ts
@@ -1,7 +1,6 @@
 import { embed, GenericEmbedded } from "./embedded";
 import { Bytes } from "./bytes";
 import { Record, Tuple } from "./record";
-import { AsPreserve } from "./symbols";
 import { Value } from "./values";
 import { Dictionary, Set } from "./dictionary";
 
@@ -27,8 +26,8 @@ export function fromJS<T = GenericEmbedded>(x: any): Value<T> {
             if (x === null) {
                 break;
             }
-            if (typeof x[AsPreserve] === 'function') {
-                return x[AsPreserve]();
+            if (typeof x.__as_preserve__ === 'function') {
+                return x.__as_preserve__();
             }
             if (Record.isRecord<Value<T>, Tuple<Value<T>>, T>(x)) {
                 return x;
diff --git a/implementations/javascript/packages/core/src/index.ts b/implementations/javascript/packages/core/src/index.ts
index ca99e59..09aa58e 100644
--- a/implementations/javascript/packages/core/src/index.ts
+++ b/implementations/javascript/packages/core/src/index.ts
@@ -1,6 +1,18 @@
 export * from './runtime';
 export * as Constants from './constants';
 
+import type { Value } from './values';
+
+declare global {
+    interface ArrayConstructor {
+        __from_preserve__<T>(v: Value<T>): undefined | Array<Value<T>>;
+    }
+}
+
+Array.__from_preserve__ = <T>(v: Value<T>) => {
+    return Array.isArray(v) ? v : void 0;
+};
+
 const _Array = Array;
 type _Array<T> = Array<T>;
 export { _Array as Array };
diff --git a/implementations/javascript/packages/core/src/runtime.ts b/implementations/javascript/packages/core/src/runtime.ts
index eaab6cc..ed9da23 100644
--- a/implementations/javascript/packages/core/src/runtime.ts
+++ b/implementations/javascript/packages/core/src/runtime.ts
@@ -16,6 +16,5 @@ export * from './merge';
 export * from './reader';
 export * from './record';
 export * from './strip';
-export * from './symbols';
 export * from './text';
 export * from './values';
diff --git a/implementations/javascript/packages/core/src/symbols.ts b/implementations/javascript/packages/core/src/symbols.ts
index 85dce93..a51f0a5 100644
--- a/implementations/javascript/packages/core/src/symbols.ts
+++ b/implementations/javascript/packages/core/src/symbols.ts
@@ -1,5 +1,23 @@
 // Symbols for various Preserves protocols.
 
-export const PreserveOn = Symbol.for('PreserveOn');
-export const AsPreserve = Symbol.for('AsPreserve');
-
+// Previously, we had the following:
+//
+// export const PreserveOn = Symbol.for('PreserveOn');
+// export const AsPreserve = Symbol.for('AsPreserve');
+// export const FromPreserve = Symbol.for('FromPreserve');
+//
+// ... and used them to define methods [PreserveOn] and [AsPreserve]
+// and functions [FromPreserve] in various namespaces. The methods
+// worked well, but the functions didn't work with TypeScript as of TS
+// 4.5 because there's no way to export a symbol-named property from a
+// `namespace`. See
+// https://github.com/microsoft/TypeScript/issues/36813.
+//
+// So, instead, I've converted them as follows:
+//
+//   [PreserveOn] ==> __preserve_on__
+//   [AsPreserve] ==> __as_preserve__
+//   [FromPreserve] ==> __from_preserve__
+//
+// and emphasised use of functions `fromJS` and `toJS` to get away
+// from the `__fiddly_details__`.