Add variable-length string and new bytes field type (#10)

Author: Copilot

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## 0.17.0-pre — 2026-04-12
+
+- Add variable-length `string` field: `byteLength` is now optional and defaults
+  to the end of the struct's buffer.
+- Add `bytes` field: returns a live `Uint8Array` view into the struct's buffer,
+  with optional fixed or variable length.
+
 ## 0.16.2 — 2026-04-05
 
 - Fix npm package imports under Node by shipping JavaScript entrypoints instead

deno.json

@@ -1,6 +1,6 @@
 {
   "name": "@rotu/structview",
-  "version": "0.16.2",
+  "version": "0.17.0-pre",
   "license": "MIT",
   "tasks": {
     "validate": "deno run -A scripts/sync-package-metadata.ts --check && deno fmt --check && deno lint && deno x vitest run && deno publish --dry-run --allow-dirty && npm pack --dry-run && npm run smoke:npm"

fields.ts

@@ -228,34 +228,61 @@ export function f64(fieldOffset: number): StructPropertyDescriptor<number> {
 }
 
 /**
- * Field for a UTF-8 fixed-length string
+ * Field for a UTF-8 string. When `byteLength` is provided, covers exactly that
+ * many bytes starting at `fieldOffset`. When omitted, extends from `fieldOffset`
+ * to the end of the struct's buffer (variable-length).
  */
 export function string(
   fieldOffset: number,
-  byteLength: number,
+  byteLength?: number,
 ): StructPropertyDescriptor<string> {
   const TEXT_DECODER = new TextDecoder()
   const TEXT_ENCODER = new TextEncoder()
   return {
     get() {
+      const end = byteLength !== undefined
+        ? fieldOffset + byteLength
+        : undefined
       const str = TEXT_DECODER.decode(
-        structBytes(this, fieldOffset, fieldOffset + byteLength),
+        structBytes(this, fieldOffset, end),
       )
       // trim all trailing null characters
       return str.replace(/\0+$/, "")
     },
     set(value) {
-      const bytes = structBytes(
-        this,
-        fieldOffset,
-        fieldOffset + byteLength,
-      )
+      const end = byteLength !== undefined
+        ? fieldOffset + byteLength
+        : undefined
+      const bytes = structBytes(this, fieldOffset, end)
       bytes.fill(0)
       TEXT_ENCODER.encodeInto(value, bytes)
     },
   }
 }
 
+/**
+ * Field for a live `Uint8Array` view into the struct's buffer. When
+ * `byteLength` is provided, covers exactly that many bytes starting at
+ * `fieldOffset`. When omitted, extends from `fieldOffset` to the end of the
+ * struct's buffer (variable-length). The field is read-only; mutations happen
+ * through the returned `Uint8Array` directly.
+ */
+export function bytes(
+  fieldOffset: number,
+  byteLength?: number,
+):
+  & StructPropertyDescriptor<Uint8Array>
+  & ReadOnlyAccessorDescriptor<Uint8Array> {
+  return {
+    get() {
+      const end = byteLength !== undefined
+        ? fieldOffset + byteLength
+        : undefined
+      return structBytes(this, fieldOffset, end)
+    },
+  }
+}
+
 /**
  * Field for a boolean stored in a byte (0 = false, nonzero = true)
  * True will be stored as 1

mod_test.ts

@@ -2,6 +2,7 @@ import {
   bigintle,
   biguintle,
   bool,
+  bytes,
   f16,
   f32,
   f64,
@@ -488,6 +489,67 @@ test("fromDataView with setter is writable and enumerable", () => {
   assert(keys.includes("val"))
 })
 
+test("string variable-length (no byteLength)", () => {
+  // 4 bytes prefix + 6 bytes for variable-length string
+  const Cls = defineStruct({
+    prefix: u32(0),
+    name: string(4),
+  })
+  const c = new Cls(new Uint8Array(10))
+  deepStrictEqual(c.name, "")
+  c.name = "hello!"
+  deepStrictEqual(c.name, "hello!")
+  // trailing nulls are trimmed
+  c.name = "hi"
+  deepStrictEqual(c.name, "hi")
+  // prefix field is unaffected
+  c.prefix = 0xdeadbeef
+  deepStrictEqual(c.prefix, 0xdeadbeef)
+  deepStrictEqual(c.name, "hi")
+})
+
+test("bytes fixed-length", () => {
+  const buf = new Uint8Array([0, 1, 2, 3, 4, 5, 6, 7])
+  const Cls = defineStruct({
+    data: bytes(2, 4),
+  })
+  const c = new Cls(buf)
+  expect(c.data).toBeInstanceOf(Uint8Array)
+  deepStrictEqual(c.data.length, 4)
+  // is a live view of the same underlying buffer
+  strictEqual(c.data.buffer, buf.buffer)
+  deepStrictEqual(c.data.byteOffset, 2)
+  // mutations through the Uint8Array are reflected in buf
+  c.data[0] = 0xff
+  deepStrictEqual(buf[2], 0xff)
+  // is read-only (no setter)
+  throws(() => {
+    // @ts-expect-error assigning to readonly property
+    c.data = new Uint8Array(4)
+  })
+})
+
+test("bytes variable-length (no byteLength)", () => {
+  const buf = new Uint8Array([10, 20, 30, 40, 50])
+  const Cls = defineStruct({
+    data: bytes(2),
+  })
+  const c = new Cls(buf)
+  expect(c.data).toBeInstanceOf(Uint8Array)
+  // extends from offset 2 to end of struct
+  deepStrictEqual(c.data.length, 3)
+  strictEqual(c.data.buffer, buf.buffer)
+  deepStrictEqual(c.data.byteOffset, 2)
+  // mutations through the Uint8Array are reflected in buf
+  c.data[1] = 0xab
+  deepStrictEqual(buf[3], 0xab)
+  // is read-only (no setter)
+  throws(() => {
+    // @ts-expect-error assigning to readonly property
+    c.data = new Uint8Array(3)
+  })
+})
+
 function hexToUint8Array(hex: string): Uint8Array {
   if (hex.length % 2 !== 0) {
     throw new TypeError("Hex input must have an even length")

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@rotu/structview",
-  "version": "0.16.2",
+  "version": "0.17.0-pre",
   "description": "Read and write structured binary data with typesafe views",
   "license": "MIT",
   "type": "module",