learn this, but not very clear

4.4this/t2.js

@@ -0,0 +1,16 @@
+let ladder = {
+  step: 0,
+  up() {
+    this.step++;
+    return this;
+  },
+  down() {
+    this.step--;
+    return this;
+  },
+  showStep: function() { // shows the current step
+    alert( this.step );
+    return this;
+  }
+};
+ladder.up().up().down().showStep().down().showStep(); // shows 1 then 0