initial project setup

src/helpers/header.ts

@@ -0,0 +1,55 @@
+// Copyright (c) Microsoft Corporation
+// Licensed under the MIT license.
+
+export interface HeaderValueParameters {
+  value: string;
+  verbatim: string;
+  params: { [k: string]: string };
+}
+
+/**
+ * Parses a header value that may contain additional parameters (e.g. `text/html; charset=utf-8`).
+ * @param headerValueText - the text of the header value to parse
+ * @returns an object containing the value and a map of parameters
+ */
+export function parseHeaderValueParameters<Header extends string | undefined>(
+  headerValueText: Header,
+): undefined extends Header ? HeaderValueParameters | undefined : HeaderValueParameters {
+  if (headerValueText === undefined) {
+    return undefined as any;
+  }
+
+  const idx = headerValueText.indexOf(";");
+  const [value, _paramsText] =
+    idx === -1
+      ? [headerValueText, ""]
+      : [headerValueText.slice(0, idx), headerValueText.slice(idx + 1)];
+
+  let paramsText = _paramsText;
+
+  // Parameters are a sequence of key=value pairs separated by semicolons, but the value may be quoted in which case it
+  // may contain semicolons. We use a regular expression to iteratively split the parameters into key=value pairs.
+  const params: { [k: string]: string } = {};
+
+  let match;
+
+  // TODO: may need to support ext-parameter (e.g. "filename*=UTF-8''%e2%82%ac%20rates" => { filename: "€ rates" }).
+  // By default we decoded everything as UTF-8, and non-UTF-8 agents are a dying breed, but we may need to support
+  // this for completeness. If we do support it, we'll prefer an ext-parameter over a regular parameter. Currently, we'll
+  // just treat them as separate keys and put the raw value in the parameter.
+  //
+  // https://datatracker.ietf.org/doc/html/rfc5987#section-3.2.1
+  while ((match = paramsText.match(/\s*([^=]+)=(?:"([^"]+)"|([^;]+));?/))) {
+    const [, key, quotedValue, unquotedValue] = match;
+
+    params[key.trim()] = quotedValue ?? unquotedValue;
+
+    paramsText = paramsText.slice(match[0].length);
+  }
+
+  return {
+    value: value.trim(),
+    verbatim: headerValueText,
+    params,
+  };
+}