Omit Content-Length header when FormData has entry with unknown length

Author: octet-stream

src/FormDataEncoder.test.ts

@@ -188,6 +188,25 @@ test("Content-Length header is read-only", t => {
   t.is(headers["Content-Length"], expected)
 })
 
+test(
+  "Does not return Content-Length header "
+    + "if FormData has entry without known length",
+
+  t => {
+    const form = new FormData()
+
+    form.set("stream", {
+      [Symbol.toStringTag]: "File",
+      name: "file.txt",
+      stream() { }
+    })
+
+    const encoder = new FormDataEncoder(form)
+
+    t.false("Content-Length" in encoder.headers)
+  }
+)
+
 test("Yields correct footer for empty FormData", async t => {
   const encoder = new FormDataEncoder(new FormData())
 
@@ -208,7 +227,7 @@ test("Returns correct length of the empty FormData content", async t => {
   const encoder = new FormDataEncoder(new FormData())
   const expected = await readStream(encoder).then(({length}) => length)
 
-  t.is<number, number>(encoder.getContentLength(), expected)
+  t.is(encoder.getContentLength(), expected)
 })
 
 test("Returns the length of the FormData content", async t => {
@@ -221,7 +240,7 @@ test("Returns the length of the FormData content", async t => {
 
   const expected = await readStream(encoder).then(({length}) => length)
 
-  t.is<number, number>(encoder.getContentLength(), expected)
+  t.is(encoder.getContentLength(), expected)
 })
 
 test(".values() yields headers as Uint8Array", t => {

src/FormDataEncoder.ts

@@ -11,14 +11,14 @@ import {isFile} from "./util/isFile.js"
 
 type FormDataEntryValue = string | FileLike
 
-type RawHeaders = Readonly<{
+interface RawHeaders {
   "Content-Type": string
-  "Content-Length": string
-}>
+  "Content-Length"?: string
+}
 
 export type FormDataEncoderHeaders =
-  & RawHeaders
-  & LowercaseObjectKeys<RawHeaders>
+  & Readonly<RawHeaders>
+  & Readonly<LowercaseObjectKeys<RawHeaders>>
 
 export interface FormDataEncoderOptions {
   /**
@@ -83,7 +83,7 @@ export class FormDataEncoder {
   /**
    * Returns Content-Length header
    */
-  readonly contentLength: string
+  readonly contentLength: string | undefined
 
   /**
    * Returns headers object with Content-Type and Content-Length header
@@ -174,12 +174,17 @@ export class FormDataEncoder {
       `${this.#DASHES}${this.boundary}${this.#DASHES}${this.#CRLF.repeat(2)}`
     )
 
-    this.contentLength = this.#getContentLength()
+    const headers: RawHeaders = {
+      "Content-Type": this.contentType
+    }
+
+    const contentLength = this.#getContentLength()
+    if (contentLength) {
+      this.contentLength = contentLength
+      headers["Content-Length"] = contentLength
+    }
 
-    this.headers = proxyHeaders(Object.freeze({
-      "Content-Type": this.contentType,
-      "Content-Length": this.contentLength
-    }))
+    this.headers = proxyHeaders(Object.freeze(headers))
 
     // Make sure following properties read-only in runtime.
     Object.defineProperties(this, {
@@ -214,17 +219,24 @@ export class FormDataEncoder {
   /**
    * Returns form-data content length
    */
-  #getContentLength(): string {
+  #getContentLength(): string | undefined {
     let length = 0
 
     for (const [name, raw] of this.#form) {
       const value = isFile(raw) ? raw : this.#encoder.encode(
         normalizeValue(raw)
       )
 
+      const size = isFile(value) ? value.size : value.byteLength
+
+      // Return `undefined` if encountered part without known size
+      if (size == null || Number.isNaN(size)) {
+        return undefined
+      }
+
       length += this.#getFieldHeader(name, value).byteLength
 
-      length += isFile(value) ? value.size : value.byteLength
+      length += size
 
       length += this.#CRLF_BYTES_LENGTH
     }
@@ -237,8 +249,8 @@ export class FormDataEncoder {
    *
    * @deprecated Use FormDataEncoder.contentLength or FormDataEncoder.headers["Content-Length"] instead
    */
-  getContentLength(): number {
-    return Number(this.contentLength)
+  getContentLength(): number | undefined {
+    return this.contentLength == null ? undefined : Number(this.contentLength)
   }
 
   /**

src/util/isFile.ts

@@ -36,8 +36,6 @@ export const isFile = (value: unknown): value is FileLike => Boolean(
     && (value as FileLike)[Symbol.toStringTag] === "File"
     && isFunction((value as FileLike).stream)
     && (value as FileLike).name != null
-    && (value as FileLike).size != null
-    && (value as FileLike).lastModified != null
 )
 
 /**