dotnet/AspNetCore.Docs · error · Error

ko.mapping.updateFromJSON, use ko.mapping.fromJSON instead.

Error message

ko.mapping.updateFromJSON, use ko.mapping.fromJSON instead. Please note that the order of parameters is different!

What it means

knockout.mapping.js throws this unconditionally when the deprecated `ko.mapping.updateFromJSON` is called. It is the JSON-string counterpart of updateFromJS and exists solely to push callers to `ko.mapping.fromJSON`, which has a different parameter order.

Source

Thrown at aspnetcore/mvc/controllers/testing/samples/3.x/TestingControllersSample/src/TestingControllersSample/wwwroot/js/knockout.mapping.js:148

			return result;
		} catch(e) {
			mappingNesting = 0;
			throw e;
		}
	};

	exports.fromJSON = function (jsonString /*, options, target*/ ) {
		var parsed = ko.utils.parseJson(jsonString);
		arguments[0] = parsed;
		return exports.fromJS.apply(this, arguments);
	};

	exports.updateFromJS = function (viewModel) {
		throw new Error("ko.mapping.updateFromJS, use ko.mapping.fromJS instead. Please note that the order of parameters is different!");
	};

	exports.updateFromJSON = function (viewModel) {
		throw new Error("ko.mapping.updateFromJSON, use ko.mapping.fromJSON instead. Please note that the order of parameters is different!");
	};

	exports.toJS = function (rootObject, options) {
		if (!defaultOptions) exports.resetDefaultOptions();

		if (arguments.length == 0) throw new Error("When calling ko.mapping.toJS, pass the object you want to convert.");
		if (exports.getType(defaultOptions.ignore) !== "array") throw new Error("ko.mapping.defaultOptions().ignore should be an array.");
		if (exports.getType(defaultOptions.include) !== "array") throw new Error("ko.mapping.defaultOptions().include should be an array.");
		if (exports.getType(defaultOptions.copy) !== "array") throw new Error("ko.mapping.defaultOptions().copy should be an array.");

		// Merge in the options used in fromJS
		options = fillOptions(options, rootObject[mappingProperty]);

		// We just unwrap everything at every level in the object graph
		return exports.visitModel(rootObject, function (x) {
			return ko.utils.unwrapObservable(x)
		}, options);
	};

View on GitHub (pinned to c67a80103a)

Solutions

1. Replace `ko.mapping.updateFromJSON(viewModel, jsonString)` with `ko.mapping.fromJSON(jsonString, {}, viewModel)` — JSON string first, then options, then the target viewModel.
2. Search the project for `updateFromJSON` and migrate every occurrence.
3. If you only have a JS object, prefer fromJS; only use fromJSON when the input is an unparsed JSON string.

Example fix

// before
ko.mapping.updateFromJSON(viewModel, jsonString);

// after
ko.mapping.fromJSON(jsonString, {}, viewModel);

// Detect legacy call and route to the supported JSON API:
function safeFromJSON(viewModel, jsonString, options) {
  return ko.mapping.fromJSON(jsonString, options || {}, viewModel);
}

function isLegacyMappingCall(name) {
  return name === 'updateFromJSON' || name === 'updateFromJS';
}

// Stubs always throw — do not catch; fix the call site.
// If you must wrap for telemetry:
try { ko.mapping.fromJSON(jsonString, {}, viewModel); }
catch (e) { /* genuine mapping error, not the deprecation stub */ }

When it happens

Trigger: Calling `ko.mapping.updateFromJSON(viewModel, jsonString)`. Any invocation at all triggers the throw because the function body is only the error.

Common situations: Legacy code that received JSON strings from an AJAX endpoint and used the old update API; partial migration where updateFromJS was renamed but the JSON variant was missed.

Related errors

• ko.mapping.updateFromJS, use ko.mapping.fromJS instead. Plea
• When calling ko.mapping.toJS, pass the object you want to co
• ko.mapping.defaultOptions().ignore should be an array.
• ko.mapping.defaultOptions().include should be an array.
• There already is an object with the key that you specified.

AI-assisted analysis of dotnet/AspNetCore.Docs@c67a80103a (2026-08-13). Data as JSON: /api/errors/b646520c71e85a32. Report an issue: GitHub.