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Posted 7 May 2014


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Replacement for ko.mapping.fromJS

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7 May 2014CPOL
There is a problem with ko mapping and here is how to fix it


About mapping business model to Knockout observable


In my app I turn my (C#) business model with JSON converter into Javascript object. Then I use ko.mapping.fromJS to turn them into observable I ran into the fact that ko.mapping is inconsistent!
(If you don't know what are observables and/or ko.mapping, have a look at Knockout)

Allow me to illustrate, say I have this C# model

class Address { string Street; }
class Person { Address Home; Address Work; }  

Which I can turn into this sample JSON data

var json = { Home: null, Work: { Street: "1 work street" } }

Which after ko.mapping will become

var kojson = ko.mapping.fromJS(json)
var json = { Home: ko.observable(), Work: { Street: ko.observable("1 work street") } }

See the problem? Home is now an observable property, whereas Work is a normal object (with observable properties)

Using the code

To solve this problem I wrote a replacement function, here is the (TypeScript) version

// unlike ko.mapping this will always map the same way, 
// i.e. all properties will be mapped to an observable (recursively)
function komapperToKO(src) {
    var mapped: Array<{ src; dst; }> = [];
    var map = function (obj) {
        obj = ko.unwrap(obj);
        if (obj === null
            || obj === undefined
            || typeof obj === "number"
            || typeof obj === "boolean"
            || typeof obj === "string"
            || typeof obj === "function"
            ) {
            return obj;
        else if (obj instanceof Array) {
            var ares = [];
            for (var i = 0; i < obj.length; i++) {
            return ko.observableArray(ares);
        else {
            var prev = mapped.enumerable().first(x => x.src === obj);
            if (prev)
                return prev.dst;
            var res = <any>{};
            mapped.push({ src: obj, dst: res });

            for (var p in obj) {
                var pv = ko.unwrap(obj[p]);
                if (typeof pv === "function") {
                    res[p] = pv;
                else if (pv instanceof Array) {
                    res[p] = map(pv);
                else {
                    res[p] = ko.observable(map(pv));
            return res;
    return map(src);

Points of Interest

A little dive inside Knockout


Keep a running update of any changes or improvements you've made here.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Super Lloyd
Software Developer (Senior)
Australia Australia
The Australia born French man who went back to Australia later in life...
Finally got over life long (and mostly hopeless usually, yay!) chronic sicknesses.
Worked in Sydney, Brisbane, Darwin, Billinudgel, Darwin and Melbourne.

Comments and Discussions

QuestionIs there a JavaScript version of the komapperToKO function? Pin
Member 109988667-Aug-14 8:47
MemberMember 109988667-Aug-14 8:47 
AnswerRe: Is there a JavaScript version of the komapperToKO function? Pin
Super Lloyd7-Aug-14 14:58
MemberSuper Lloyd7-Aug-14 14:58 

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