work

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

@@ -3,22 +3,50 @@
 
 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.
+But we left a 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).
+For objects, there's a special additional conversion 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:
+For some built-in objects it is implemented in special way, but mostly comes in two flavors:
 
-- string
-- number
+- `ToPrimitive(obj, "string")` for a conversion to string
+- `ToPrimitive(obj, "number")` for a conversion to number
 
-TODO
+So, if we convert an object to string, then first `ToPrimitive(obj, "string")` is applied, and then the resulting primitive is converted using primitive rules. The similar thing for a numeric conversion.
+
+What's most interesting in `ToPrimitive` is its customizability.
+
+## toString and valueOf
+
+`ToPrimitive` is customizable via methods `toString()` and `valueOf()`.
+
+The general algorithm of `ToPrimitive(obj, "string")` is:
 
 
+1. Call the method `obj.toString()` if it exists.
+2. If the result is a primitive, return it.
+3. Call the method `obj.valueOf()` if it exists.
+4. If the result is a primitive, return it.
+5. Otherwise `TypeError` (conversion failed)
 
-The method `toString` is automatically called by Javascript when the object is converted to a string:
+
+The `ToPrimitive(obj, "number")` is the same, but `valueOf()` and `toString()` are swapped:
+
+1. Call the method `obj.valueOf()` if it exists.
+2. If the result is a primitive, return it.
+3. Call the method `obj.toString()` if it exists.
+4. If the result is a primitive, return it.
+5. Otherwise `TypeError` (conversion failed)
+
+```smart header="ToPrimitive returns a primitive"
+As we can see, the result of `ToPrimitive` is always a primitive, because even if `toString/valueOf` return a non-primitive value, it is ignored.
+
+But it can be any primitive. There's no control whether `toString()` returns exactly a string or, say a boolean.
+```
+
+Let's see an example. Here we implement our own string conversion for `user`:
 
 ```js run
 let user = {
@@ -41,6 +69,93 @@ alert( user );  // User John
 Looks much better than the default `[object Object]`, right?
 
 
+Now let's add a custom numeric conversion with `valueOf`:
+
+```js run
+let user = {
+
+  name: 'John',
+  age: 30,
+
+*!*
+  valueOf() {
+    return this.age;
+  }
+*/!*
+
+};
+
+*!*
+alert( +user );  // 30
+*/!*
+```
+
+In most projects though, only `toString()` is used, because objects are printed out (especially for debugging) much more often than added/substracted/etc.
+
+If only `toString()` is implemented, then both string and numeric conversions use it.
+
+## Examples for built-ins
+
+Let's check a few examples to finally get the whole picture.
+
+```js run
+alert( [] + 1 ); // '1'
+alert( [1] + 1 ); // '11'
+alert( [1,2] + 1 ); // '1,21'
+```
+
+The array from the left side of `+` is first converted to primitive using `toPrimitive(obj, "number")`.
+
+For arrays (and most other built-in objects) only `toString` is implemented, and it returns a list of items.
+
+So we'll have:
+
+```js 
+alert( '' + 1 ); // '1'
+alert( '1' + 1 ); // '11'
+alert( '1,2' + 1 ); // '1,21'
+```
+
+Now the addition has the first operand -- a string, so it converts the second one to a string also. Hence the result.
+
+Now with a plain object:
+
+```js run
+alert( +{} ); // NaN
+alert( {} + {} ); // [object Object][object Object]
+```
+
+Plain objects actually have both `toString()` and `valueOf()`:
+
+................TODO ALG OF OBJECT TOSTRING
+
+The result of these operations should be somewhat obvious now.
+
+
+
+
+
+
+## [[Class]]
+
+From the chapter <info:types> we know that `typeof` cannot distinguish different kinds of objects. Arrays, plain objects and others are all the same "object" for it.
+
+But there's a semi-hidden way to access the right class.
+
+Most built-in objects 
+
+
+
+
+Here are some built-in objects
+
+Most built-in object implement only `toString()`. From the algorithm string conversion is much more widely used
+
+
+
+
+
+
 The similar thing with the method `valueOf`. It is called when the object is converted to a number.
 
 ```js run