diff --git a/src/progress-stream.js b/src/progress-stream.js
index 3440eb148..b14d62cdd 100644
--- a/src/progress-stream.js
+++ b/src/progress-stream.js
@@ -4,39 +4,46 @@ const stream = require('stream'),
     TransformStream = stream.Transform;
 
 class ProgressStream extends TransformStream {
+    #options;
+    #transferred;
+    #delta;
+    #started;
+    #startTime;
+    #interval;
+
     constructor(options) {
         super();
-        this._options = Object.assign({ interval: 10 * 1000 }, options);
-        this._transferred = 0;
-        this._delta = 0;
-        this._started = false;
-        this._startTime = null;
-        this._interval = null;
+        this.#options = Object.assign({ interval: 10 * 1000 }, options);
+        this.#transferred = 0;
+        this.#delta = 0;
+        this.#started = false;
+        this.#startTime = null;
+        this.#interval = null;
     }
 
     stats() {
-        const totalSecs = Math.round((Date.now() - this._startTime)/1000);
-        return { startTime: this._startTime, totalSecs, transferred: this._transferred };
+        const totalSecs = Math.round((Date.now() - this.#startTime)/1000);
+        return { startTime: this.#startTime, totalSecs, transferred: this.#transferred };
     }
 
     _start() {
-        this._startTime = Date.now();
-        this._started = true;
-        this._interval = setInterval(() => {
-            const speed = this._delta * 1000 / this._options.interval;
-            this._delta = 0;
-            this.emit('progress', { speed, transferred: this._transferred });
-        }, this._options.interval);
+        this.#startTime = Date.now();
+        this.#started = true;
+        this.#interval = setInterval(() => {
+            const speed = this.#delta * 1000 / this.#options.interval;
+            this.#delta = 0;
+            this.emit('progress', { speed, transferred: this.#transferred });
+        }, this.#options.interval);
     }
 
     _stop() {
-        clearInterval(this._interval);
+        clearInterval(this.#interval);
     }
 
     _transform(chunk, encoding, callback) {
-        if (!this._started) this._start();
-        this._transferred += chunk.length;
-        this._delta += chunk.length;
+        if (!this.#started) this._start();
+        this.#transferred += chunk.length;
+        this.#delta += chunk.length;
         callback(null, chunk);
     }