That's because prefixing something with an underscore is a convention and not a language feature. In order to truly protect object properties you need visibility, as pointed by Sammy and explained in my latest reply to my own comment.
If the language doesn't have visibility features any method you use to try and protect the properties will simply be an ugly jerry-rig (that includes the prefix).
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Most often _ is prefixed before the properties to signify private properties but they can still be modified outside the class.
Thanks for the feedback and I will look into Private Fields
That's because prefixing something with an underscore is a convention and not a language feature. In order to truly protect object properties you need visibility, as pointed by Sammy and explained in my latest reply to my own comment.
If the language doesn't have visibility features any method you use to try and protect the properties will simply be an ugly jerry-rig (that includes the prefix).