feat(stream): add ifEmpty option to Stream.join

deno_dist/stream/stream/constructors.ts

@@ -142,7 +142,8 @@ class EmptyStream<T = any> extends StreamBase<T> implements Stream<T> {
   intersperse(): Stream<T> {
     return this;
   }
-  join({ start = '', end = '' } = {}): string {
+  join({ start = '', end = '', ifEmpty = undefined } = {}): string {
+    if (undefined !== ifEmpty) return ifEmpty;
     return start.concat(end);
   }
   mkGroup({

deno_dist/stream/stream/interface.ts

@@ -509,6 +509,7 @@ export interface Stream<T> extends FastIterable<T>, Streamable<T> {
    * * start: (optional) a start string to prepend at the start
    * * end: (optional) an end string to append at the end
    * * valueToString: (default: String) a function converting a Stream element to a string
+   * * ifEmpty: (optional) a string to return instead of the start and end tag if the stream is empty
    * @example
    * Stream.of(1, 2, 3).join({ start: '<', sep: ', ', end: '>' })
    * // => '<1, 2, 3>'
@@ -519,6 +520,7 @@ export interface Stream<T> extends FastIterable<T>, Streamable<T> {
     start?: string;
     end?: string;
     valueToString?: (value: T) => string;
+    ifEmpty?: string;
   }): string;
   /**
    * Returns a Stream starting with `options.sep`, then returning the elements of this Stream interspersed with `options.sep`, and ending with

deno_dist/stream/stream/stream-custom.ts

@@ -400,12 +400,16 @@ export abstract class StreamBase<T> implements Stream<T> {
     start = '',
     end = '',
     valueToString = String,
+    ifEmpty = undefined,
   } = {}): string {
     const done = Symbol('Done');
     const iterator = this[Symbol.iterator]();
     let value: T | typeof done = iterator.fastNext(done);
 
-    if (done === value) return start.concat(end);
+    if (done === value) {
+      if (undefined !== ifEmpty) return ifEmpty;
+      return start.concat(end);
+    }
 
     let result = start.concat(valueToString(value));