learn this, but not very clear

4.4this/t1.js

@@ -0,0 +1,18 @@
+let calculator={
+    a: 0,
+    b: 0,
+    read(){
+        this.a = prompt("please type a");
+        this.b = prompt("please type b");
+    },
+    sum(){
+        return Number(this.a) + Number(this.b);
+    },
+    mul(){
+        return this.a * this.b;
+    }
+};
+
+calculator.read();
+alert(calculator.sum());
+alert(calculator.mul());