add: implement peek and remove

Author: qiao

src/immutable_sequence.js

@@ -78,6 +78,22 @@
     return split[1].peekFirst();
   };
 
+  /**
+   * Get the first element in the sequence.
+   * @return {*}
+   */
+  ImmutableSequence.prototype.peekFirst = function () {
+    return this.tree_.peekFirst();
+  };
+
+  /**
+   * Get the last element in the sequence.
+   * @return {*}
+   */
+  ImmutableSequence.prototype.peekLast = function () {
+    return this.tree_.peekLast();
+  };
+
   /**
    * Add an element to the front of the sequence.
    * @param {*} v
@@ -96,5 +112,21 @@
     return new ImmutableSequence(this.tree_.addLast(v));
   };
 
+  /**
+   * Remove an element from the front of the sequence.
+   * @return {ImmutableSequence}
+   */
+  ImmutableSequence.prototype.removeFirst = function () {
+    return new ImmutableSequence(this.tree_.removeFirst());
+  };
+
+  /**
+   * Remove an element from the end of the sequence.
+   * @return {ImmutableSequence}
+   */
+  ImmutableSequence.prototype.removeLast = function () {
+    return new ImmutableSequence(this.tree_.removeLast());
+  };
+
   return ImmutableSequence;
 }));