I want to determine why Knockout.js custom binding called update

ko.bindingHandlers.MyBind={  
    update:function(element, valueAccessor){

        // Obtain the current value of the binding
        varobj=ko.unwrap(valueAccessor());
        value = ko.unwrap(obj.value);
        var size = ko.unwrap(obj.size);

        // retrieve the previously saved value
        varp=element.__MyBindPreviousValue;

        // TODO: Process with current and previous values
        element.innerText=`${value},${size}<=(${p?p.value:'},${p?p.size:'}')`;

        // save this time's value in the right place
        // Here we simply set the value for element.
        element.__MyBindPreviousValue={value:value,size:size};
    } 
};