String methods not recognizable for character in QML function

import QtQuick 
Window {
    id: window
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")
    Component.onCompleted:
    {
        let s = "hello"
        console.log(s[0].toLowerCase())
        //console.log(s.toLowerCase()) // this is fine 
    }
}