How do you check if a property is undefined in qml?

Question

How do you check if a property is undefined in qml?

This is what I am trying to do:

Button {
    id: myButton
    text: if (text === "undefined"){"default text"}
}

Solution 1

Try: text: text ? text : "default text"

"undefined" is just a string representation of a reference not referencing anything, just like None, or NULL in other languages.

=== is strict comparison operator, you might want to read this thread: https://stackoverflow.com/questions/523643/difference-between-and-in-javascript

Solution 2

import QtQuick 2.3
import QtQuick.Controls 1.2

Button {
    id: myButton
    text: text ? text : "default text"
}

This answer throws a warning for me.

QML Button: Binding loop detected for property "text"

Changing text to modelText instead throws an error.

ReferenceError: modelText is not defined

This stops the Javascript execution for me; i.e. the next line isn't called.

Via Javascript

The same happens when setting it via Javascript, but is quite verbose.

import QtQuick 2.3
import QtQuick.Controls 1.2

Button {
    id: myButton
    text: "default text"

    Component.onCompleted: {
        if (modelText !== "undefined") {
            myButton.text = modelText;
        }
    }
}

Using typeof

The typeof operator mutes the error and works as expected.

import QtQuick 2.3
import QtQuick.Controls 1.2

Button {
    id: myButton
    text: "default text"

    Component.onCompleted: {
        if (typeof modelText !== "undefined") {
            myButton.text = modelText;
        }
    }
}

Solution 3

To compare with undefined you write text === undefined. This will evaluate to false if text is null.

If you want check if value is present (i.e., check for both undefined and null), use it as condition in if statement or ternary operator. If you need to store result of comparison as a boolean value, use var textPresent = !!text (though double ! might appear confusing to one reading the code).

Comments

[Anon]

That was a very clever solution. Thanks. Just to explain to anyone else; its sort of like saying (correct me if I am wrong) if (text === text){text} else{"default text"}

[Kissiel]

if (text) { text } else {"default text"} to be exact. if (object) evaluates to false if object is undefined. Similar hack to C-style if(pointer) that evaluates to false if pointer has a value of 0 (NULL). It's worth noting that text variable used for text property of a button is taken from outside scope. It'll be much clearer with: text: inText ? inText : "default text", or if(inText) { text } else {"default text"}

[Anon]

Sorry to be daft, but this is something I never quite understood. In logic, it would technically read as this - if (text is true) then {text = text} else {text = "default text"} -- is this accurate?

[Kissiel]

You're pretty much right. The only unintuitive thing about this pseudo-code is if (text is true). I find it easier to think as if (text *is*) or if (text exists). Another good source: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

[RvdK]

This will fail with text = "" (empty string), the if will return false