Add normalizeValue helper that replaces all CR and LF in non-File field's value following the spec.

Author: octet-stream

lib/Encoder.ts

@@ -1,6 +1,7 @@
 import createBoundary from "./util/createBoundary"
-import escape from "./util/escapeName"
+import normalize from "./util/normalizeValue"
 import isFormData from "./util/isFormData"
+import escape from "./util/escapeName"
 import isFile from "./util/isFile"
 
 import {FormDataLike} from "./FormDataLike"
@@ -119,7 +120,7 @@ export class FormDataEncoder {
 
       length += isFile(value)
         ? value.size
-        : this.#encoder.encode(String(value)).byteLength
+        : this.#encoder.encode(normalize(value)).byteLength
 
       length += this.#CRLF_BYTES_LENGTH
     }
@@ -167,7 +168,7 @@ export class FormDataEncoder {
     for (const [name, value] of this.#form.entries()) {
       yield this.#getFieldHeader(name, value)
 
-      yield isFile(value) ? value : this.#encoder.encode(String(value))
+      yield isFile(value) ? value : this.#encoder.encode(normalize(value))
 
       yield this.#CRLF_BYTES
     }

lib/util/escapeName.ts

@@ -1,3 +1,10 @@
+/**
+ * Escape fieldname following the spec requirements.
+ *
+ * See: https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#multipart-form-data
+ *
+ * @param name A fieldname to escape
+ */
 const escapeName = (name: unknown) => String(name)
   .replace(/\r/g, "%0D") // CR
   .replace(/\n/g, "%0A") // LF

lib/util/normalizeValue.test.ts

@@ -0,0 +1,23 @@
+import test from "ava"
+
+import normalize from "./normalizeValue"
+
+const expected = "First line.\r\nSecond line.\r\nThird line."
+
+test("Replaces all CR not followed by LF with CRLF", t => {
+  const actual = normalize("First line.\rSecond line.\rThird line.")
+
+  t.is(actual, expected)
+})
+
+test("Replaces all LF not predicessed by CR with CRLF", t => {
+  const actual = normalize("First line.\nSecond line.\nThird line.")
+
+  t.is(actual, expected)
+})
+
+test("Keeps all CRLF without changes", t => {
+  const actual = normalize(expected)
+
+  t.is(actual, expected)
+})

lib/util/normalizeValue.ts

@@ -0,0 +1,11 @@
+/**
+ * Normalize non-File value following the spec requirements.
+ *
+ * See: https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#multipart-form-data
+ *
+ * @param value A value to normalize
+ */
+const normalizeValue = (value: unknown): string => String(value)
+  .replace(/\r(?!\n)|(?<!\r)\n/g, "\r\n")
+
+export default normalizeValue