Questions about Knockout Mapping

[lzl124631x]

knockout/knockout#1895

[lzl124631x]

var data = { a: { b: 1} };
var vm = ko.mapping.fromJS(data);
console.log(vm.a.b());

Why vm.a is not an observable? http://knockoutjs.com/documentation/plugins-mapping.html#how-things-are-mapped

[lzl124631x]

https://jsfiddle.net/Richard_Liu/pauzy5wL/

Why error occurs in the 3rd case?

[lzl124631x]

Copying certain properties using “copy”

When converting your view model back to a JS object, by default the mapping plugin will create observables based on the rules explained above.

The bold part is wrong! Shouldn't it be 'When converting your JS object to view model'?

[lzl124631x]

The array copy can be used for efficiency to copy array or object properties including children. If an >array or object property is not specified in copy or observe then it is recursively mapped:

var data = {
   a: "a",
   b: [{ b1: "v1" }, { b2: "v2" }] 
};

var result = ko.mapping.fromJS(data, { observe: "a" });
var result2 = ko.mapping.fromJS(data, { observe: "a", copy: "b" }); //will be faster to map.

Why doesn't knockout work in this way: if observe is specified and doesn't include the array b, b is always copied directly?

[lzl124631x]

console.clear();
var data = [
    { a: "a1", b: "b1" },
    { a: "a2", b: "b2" }
];

var m1 = ko.mapping.fromJS(data, { 'observe' : '[0].a' }); // can only maps the property `a` in the first element
console.log(m1());

How to only observe property a of elements in array? Is there some regex-like pattern?

---

I've to use the following mapping?

var mapping = {
    create: function(options) {
        return ko.mapping.fromJS(options.data, { 'observe': 'a' });
    }
}
var m2 = ko.mapping.fromJS(data, mapping);

[lzl124631x]

http://jsfiddle.net/Richard_Liu/vfptk976/

ko.mapping.fromJS(data, { observe: 'b' }, data); doesn't work as expected, is this a bug or designed intentionally?

[lzl124631x]

https://jsfiddle.net/Richard_Liu/goycbmt9/

var data = [
        { id : 1, name : 'Alice' },
        { id : 2, name : 'Bob' }
    ];
var mapping = {
    'create': function(options) {
        return options.data;
    }
}
var m1 = ko.observableArray();
ko.mapping.fromJS(data, mapping, m1);
console.log(m1.push) // m1 is an observableArray!
var m2 = ko.observable();
ko.mapping.fromJS(data, mapping, m2);
console.log(m2.push); // m2 is an observable!

So the initial type (observable or observableArray) of view model matters?

I know the right way is to use m = ko.mapping.fromJS(...) for the first time and then ko.mapping.fromJS(..., m) later on. I just think it's tiresome and hope always calling ko.mapping.fromJS(..., m) will do the work, no need to do special work at the first time.

[lzl124631x]

Hi there?

[KantJoe]

@lzl124631x using options by the way m1 should get observe.

[lzl124631x]

Do you mean that, in my latest question, m1 which is initialized as an observableArray, will still be an observable after ko.mapping.fromJS?