Comma

This warning category is spelled [comma] by qmllint.

Do not use comma expressions

What happened?

A JavaScript comma expression was used outside of a for loop.

Why is this bad?

Comma expressions reduce readability of the code and obscure side-effects.

Example

 import QtQuick

 Item {
     Component.onCompleted: init(config, true), enableLogging(categories), run(1000) // millis
 }

To fix this warning, refactor the code to use distinct statements for each operation. This way, each side effect is explicit instead of happening as part of another unrelated operation:

 import QtQuick

 Item {
     Component.onCompleted: {
         init(config, true)
         enableLogging(categories)
         run(1000) // millis
     }
 }

In addition, there are some special considerations for cases where comma expressions appear because a variable is intentionally being captured for a binding, as in the following code, where previewOfFirstPage is a function defined in C++ which internally depends on Config.fontSize:

 Text {
    // This causes the function to re-run when fontSize changes
    text: Config.fontSize, documentProvider.previewOfFirstPage()
 }

If you encounter this situation, consider one of the following approaches:

• If a function such as previewOfFirstPage depends on a property, prefer making this dependency explicit by passing the value as an argument.

   Text {
      text: documentProvider.previewOfFirstPage(Config.fontSize)
   }
  

• If changing the function signature is undesirable for API reasons, consider replacing the function with a Q_PROPERTY instead, so that change notifications for it can be emitted when the depenency is modified in C++:

   void Config::setFontSize(int fontSize) {
      if (m_fontSize == fontSize)
         return;
      m_fontSize = fontSize;
      emit fontSizeChanged();
      emit previewOfFirstPageChanged();
   }
  

   Text {
      text: documentProvider.previewOfFirstPage
   }
  

• If modifying the C++ implementation or adding QML dependencies is not possible, use a qmllint directive to silence the warning. Include a comment explaining that the intention is to capture the variable in the binding.

   Text {
      // This causes the function to re-run when fontSize changes
      text: Config.fontSize, documentProvider.previewOfFirstPage() // qmllint disable comma
   }