DataWeave has some very nice features to transform data objects. One of those nice features is the update operator. With the update operator we can change values of keys in an object using a very concise syntax. We don’t have to go through all keys and create a new object, but we can pinpoint the exact key and change the value. To get the correct key we use selectors. Once we have the key we can set a new value. We can define a variable to contain the current value if we want to use it to define a new value. Also is it possible to add a condition that needs to be true to change the value. Finally the update operator supports upserting a value if the key might not exist yet.

The complete syntax of the update operator is as follows:

<object> update {
    case <optional variable> at <key selector> <optional condition if (...)> -> <new value>

In the following example we see several use cases of the update operator:


%dw 2.0

var obj = {
    user: {
        firstName: "Hubert",
        lastName: "Klein Ikkink"
    alias: "mrhaki",
    country: "NL"

output application/json
obj update {
    // Simply use . selector to get key and set new value
    case .alias ->  "haki"

    // Using ! to upsert a key if doesn't exist yet.
    case .likes! -> ["Clojure", "DataWeave", "Groovy"]

    // We can use a variable for the selector value
    // and use it for a new value
    case firstName at .user.firstName -> firstName[0] ++ ".A."

    // DataWeave always provides $ as variable if we don't use at.
    // Here we can use $ to define a new value.
    case .user.lastName -> $ splitBy " " map ((item) -> item[0]) joinBy ""

    // We can add a condition for which we want to update the key
    // with a new value. In this example if country is "NL"
    // the value is "Netherlands", for other values of country
    // no transformation happens.
    case country at .country if (country == "NL") ->  "Netherlands"


    "user": {
        "firstName": "H.A.",
        "lastName": "KI"
    "alias": "haki",
    "country": "Netherlands",
    "likes": [

Written with DataWeave 2.4.