Proxy encoder headers so that can be accessed with case-insensitive manner.

Author: octet-stream

src/FormDataEncoder.test.ts

@@ -131,12 +131,63 @@ test("contentLength property cannot be deleted", t => {
 test("Has correct headers", async t => {
   const encoder = new FormDataEncoder(new FormData())
 
-  t.deepEqual(encoder.headers, {
+  t.deepEqual({...encoder.headers}, {
     "Content-Type": `multipart/form-data; boundary=${encoder.boundary}`,
     "Content-Length": await readStream(encoder).then(({length}) => `${length}`)
   })
 })
 
+test("Headers can be accessed by lowercased keys", async t => {
+  const encoder = new FormDataEncoder(new FormData())
+
+  t.true("content-type" in encoder.headers)
+  t.is(
+    encoder.headers["content-type"],
+
+    `multipart/form-data; boundary=${encoder.boundary}`
+  )
+
+  t.true("content-length" in encoder.headers)
+  t.is(
+    encoder.headers["content-length"],
+
+    await readStream(encoder).then(({length}) => `${length}`)
+  )
+})
+
+test("FormDataEncoder.headers property is read-only", t => {
+  const encoder = new FormDataEncoder(new FormData())
+
+  const expected = {...encoder.headers}
+
+  // @ts-expect-error
+  try { encoder.headers = {foo: "foo"} } catch { /* noop */ }
+
+  t.deepEqual({...encoder.headers}, expected)
+})
+
+test("Content-Type header is read-only", t => {
+  const {headers} = new FormDataEncoder(new FormData())
+
+  const expected = headers["Content-Type"]
+
+  // @ts-expect-error
+  try { headers["Content-Type"] = "can't override" } catch { /* noop */ }
+
+  t.is(headers["Content-Type"], expected)
+})
+
+test("Content-Length header is read-only", t => {
+  const {headers} = new FormDataEncoder(new FormData())
+
+  const expected = headers["Content-Length"]
+
+  // @ts-expect-error
+  try { headers["Content-Length"] = "can't override" } catch { /* noop */ }
+
+  t.is(headers["Content-Length"], expected)
+})
+
 test("Yields correct footer for empty FormData", async t => {
   const encoder = new FormDataEncoder(new FormData())
 

src/FormDataEncoder.ts

@@ -5,11 +5,22 @@ import escape from "./util/escapeName.js"
 
 import {isFile} from "./util/isFile.js"
 import {isFormData} from "./util/isFormData.js"
+import {proxyHeaders} from "./util/proxyHeaders.js"
+import type {LowercaseObjectKeys} from "./util/LowercaseObjectKeys.js"
 import {FormDataLike} from "./FormDataLike.js"
 import {FileLike} from "./FileLike.js"
 
 type FormDataEntryValue = string | FileLike
 
+type RawHeaders = Readonly<{
+  "Content-Type": string
+  "Content-Length": string
+}>
+
+export type FormDataEncoderHeaders =
+  & RawHeaders
+  & LowercaseObjectKeys<RawHeaders>
+
 export interface FormDataEncoderOptions {
   /**
    * When enabled, the encoder will emit additional per part headers, such as `Content-Length`.
@@ -76,10 +87,7 @@ export class FormDataEncoder {
   /**
    * Returns headers object with Content-Type and Content-Length header
    */
-  readonly headers: {
-    "Content-Type": string
-    "Content-Length": string
-  }
+  readonly headers: FormDataEncoderHeaders
 
   /**
    * Creates a multipart/form-data encoder.
@@ -167,10 +175,10 @@ export class FormDataEncoder {
 
     this.contentLength = String(this.#getContentLength())
 
-    this.headers = Object.freeze({
+    this.headers = proxyHeaders(Object.freeze({
       "Content-Type": this.contentType,
       "Content-Length": this.contentLength
-    })
+    }))
 
     // Make sure following properties read-only in runtime.
     Object.defineProperties(this, {

src/util/LowercaseObjectKeys.ts

@@ -0,0 +1,4 @@
+// Baed on: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/5336057c43fcd14eabe7ae8536b51a7c7b2b21bf/types/lowercase-object-keys/index.d.ts
+export type LowercaseObjectKeys<T extends object> = {
+  [K in keyof T as K extends string ? Lowercase<K> : K]: T[K]
+}

src/util/proxyHeaders.ts

@@ -0,0 +1,29 @@
+import type {LowercaseObjectKeys} from "./LowercaseObjectKeys.js"
+
+type AnyObject = Record<string | symbol, string>
+
+function getProperty<T extends AnyObject>(
+  target: T, prop: string | symbol
+): string | undefined {
+  if (typeof prop !== "string") {
+    return target[prop]
+  }
+
+  for (const [name, value] of Object.entries(target)) {
+    if (prop.toLowerCase() === name.toLowerCase()) {
+      return value
+    }
+  }
+
+  return undefined
+}
+
+export const proxyHeaders = <T extends AnyObject>(target: T) => new Proxy(
+  target,
+
+  {
+    get: (target, prop) => getProperty(target, prop),
+
+    has: (target, prop) => getProperty(target, prop) !== undefined
+  }
+) as T & LowercaseObjectKeys<T>