equality-type-coercion.qdoc

Go to the documentation of this file.

// Copyright (C) 2026 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
\ingroup qmllint-warnings-and-errors
 
\title Loose equality comparison type coercion
\brief [equality-type-coercion] Using loose equality comparison operators may coerce values.
 
\qmllintwarningcategory equality-type-coercion
 
\section1 Loose equality comparison type coercion
 
\section2 What happened?
Two values were compared for equality using the loose comparison operators.
 
\section2 Why is that bad?
The loose comparison operators can coerce values to a different type before
checking for equality. This can lead to unexpected results.
 
\section2 Example
Here is an example with a list of Rectangles and a TextInput. When the user
enters a number, the Rectangle at that index is highlighted.
The code has a flaw. If the input is empty, the first rectangle is highlighted
in red, because "" == 0.
\qml
import QtQuick
 
Item {
    TextInput {
        id: input
    }
    Repeater {
        model: 3
        Rectangle {
            // first rectangle is red on empty input
            color: input.text == index ? "red" : "blue"
        }
    }
}
\endqml
In general, use the strict comparison operators \c{===} and \c{!==}.
Even if you are aware of the coercion, it is still recommended to use explicit
casts and strict comparisons instead.
\qml
import QtQuick
 
Item {
    TextInput {
        id: input
    }
    Repeater {
        model: 3
        Rectangle {
            // check inputs, use explicit casts, and strict equality operators
            color: input.text.length !== 0
                   && Number(input.text) === index ? "red" : "blue"
        }
    }
}
\endqml
*/