Object Read-Only Properties

Question

I was reading this question about read-only properties, and I came upon this snippet:

var myObject = {
    get readOnlyProperty() { return 42; }
};

alert(myObject.readOnlyProperty); // 42
myObject.readOnlyProperty = 5;    // Assignment is allowed, but doesn't do anything
alert(myObject.readOnlyProperty); // 42

Now, I know to hide the scope, you can use an IIFE, to also make a variable or property "private", but what I don't understand is:

Why is assignment allowed, and if it's allowed, how can nothing happen? In this snippet there is no inferred scope, so I don't understand how something in JS can infer a private property.

Answer 1

Why is assignment allowed, and if it's allowed, how can nothing happen? In this snippet there is no inferred scope, so I don't understand how something in JS can infer a private property.

Because assignments (before strict mode) never throw and making it throw would violate that invariant that people expect. While you can still override it (by making a setter and making that throw) this is the default behavior in JavaScript for properties. We don't like it but it is what it is.

If you use strict mode - you should get:

TypeError: setting a property that has only a getter