diff --git a/implementations/javascript/packages/schema/src/compiler.ts b/implementations/javascript/packages/schema/src/compiler.ts
index 969047f..8b6faee 100644
--- a/implementations/javascript/packages/schema/src/compiler.ts
+++ b/implementations/javascript/packages/schema/src/compiler.ts
@@ -17,7 +17,7 @@ export type CompileEnvEntry = {
 export function compile(env: Array<CompileEnvEntry>, schema: Schema, preservesModule = '@preserves/core'): string {
     const literals = new Dictionary<string, never>();
     const types: Array<Item> = [];
-    const predicates: Array<Item> = [];
+    const functions: Array<Item> = [];
     let temps: Array<string> = [];
 
     function environmentWith<R>(name: symbol,
@@ -234,12 +234,20 @@ export function compile(env: Array<CompileEnvEntry>, schema: Schema, preservesMo
         }
         types.push(
             seq(`export type ${name.description!} = `, typeFor(pattern), `;`));
-        predicates.push(
+        functions.push(
             seq('export function ', `is${name.description!}`,
                 '(v: any): v is ', name.description!, ' ',
                 block(
                     ... temps.length > 0 ? [seq('let ', commas(... temps), ': any')] : [],
                     seq('return ', recognizer))));
+        functions.push(
+            seq('export function ', `as${name.description!}`,
+                '(v: any): ', name.description!, ' ',
+                block(
+                    seq(`if (!is${name.description!}(v)) `,
+                        block(`throw new TypeError("${name.description!}")`),
+                        ' else ',
+                        block(`return v`)))));
     }
 
     const f = new Formatter();
@@ -260,7 +268,7 @@ export function compile(env: Array<CompileEnvEntry>, schema: Schema, preservesMo
     });
     f.newline();
 
-    predicates.forEach(p => {
+    functions.forEach(p => {
         f.write(p);
         f.newline();
         f.newline();
diff --git a/implementations/javascript/packages/schema/src/reader.ts b/implementations/javascript/packages/schema/src/reader.ts
index 24501c7..cce4362 100644
--- a/implementations/javascript/packages/schema/src/reader.ts
+++ b/implementations/javascript/packages/schema/src/reader.ts
@@ -54,9 +54,7 @@ export function parseSchema(toplevelTokens: Array<Input>): Schema {
             }
             definitions.set(name, parseDefinition(name, clause.slice(2).map(peel)));
         } else if (clause.length === 2 && is(clause[0], M.$version)) {
-            // TODO: use asVersion
-            if (!M.isVersion(clause[1])) invalidClause(clause);
-            version = clause[1];
+            version = M.asVersion(clause[1]);
         } else {
             invalidClause(clause);
         }
diff --git a/implementations/javascript/packages/schema/src/schemaschema.ts b/implementations/javascript/packages/schema/src/schemaschema.ts
index 9351797..5dec04b 100644
--- a/implementations/javascript/packages/schema/src/schemaschema.ts
+++ b/implementations/javascript/packages/schema/src/schemaschema.ts
@@ -110,8 +110,12 @@ export function isSchema(v: any): v is Schema {
     );
 }
 
+export function asSchema(v: any): Schema {if (!isSchema(v)) {throw new TypeError("Schema");} else {return v;};}
+
 export function isVersion(v: any): v is Version {return v === $1;}
 
+export function asVersion(v: any): Version {if (!isVersion(v)) {throw new TypeError("Version");} else {return v;};}
+
 export function isPattern(v: any): v is Pattern {
     return (
         (
@@ -222,6 +226,8 @@ export function isPattern(v: any): v is Pattern {
     );
 }
 
+export function asPattern(v: any): Pattern {if (!isPattern(v)) {throw new TypeError("Pattern");} else {return v;};}
+
 export function isNamedPattern(v: any): v is NamedPattern {
     return (
         (
@@ -233,4 +239,8 @@ export function isNamedPattern(v: any): v is NamedPattern {
     );
 }
 
+export function asNamedPattern(v: any): NamedPattern {
+    if (!isNamedPattern(v)) {throw new TypeError("NamedPattern");} else {return v;};
+}
+