work

1-js/2-first-steps/21-object-tostring-valueof/article.md

@@ -0,0 +1,63 @@
+
+# Object to primitive conversion
+
+In the chapter <info:type-conversions> we've seen the rules for numeric, string and boolean conversions.
+
+But we left the gap for objects. Now let's close it.
+
+[cut]
+
+The operation that converts an object to a primitive is called [ToPrimitive](https://tc39.github.io/ecma262/#sec-toprimitive).
+
+Some build-in language objects have their own implementation of it, but for most objects, including our own, it comes in two flavours:
+
+- string
+- number
+
+TODO
+
+
+
+The method `toString` is automatically called by Javascript when the object is converted to a string:
+
+```js run
+let user = {
+
+  name: 'John',
+
+*!*
+  toString() {
+    return `User ${this.firstName}`;
+  }
+*/!*
+
+};
+
+*!*
+alert( user );  // User John
+*/!*
+```
+
+Looks much better than the default `[object Object]`, right?
+
+
+The similar thing with the method `valueOf`. It is called when the object is converted to a number.
+
+```js run
+let room = {
+  number: 777,
+
+  valueOf() {
+    return this.number; 
+  },
+};
+
+alert( +room );  // 777, valueOf is called
+```
+
+What really sounds strange -- is the name of the method. Why is it called "valueOf", why not "toNumber"?
+
+The reason is that `valueOf` is used by default to convert an object to a primitive for operations where a primitive value is required.
+
+
+