[Buffer] length is not a readonly attribute

[haoxli]

readonly attribute unsigned long length;

In buffer.md, the length of Buffer should be readonly, but actually it can be modified.

var buff = new Buffer(8);
var original = buff.length;
buff.length = buff.length + 1;
print(original);
print(buff.length);

The print result is 9, not the expected 8.

[jimmy-huang]

this is probably a limitation of jerryscript, there's not a way for us to set a read-only attribute on any js objects, at least I am not aware of any, @brianjjones?

[pfalcon]

Quick grep over JrS codebase:

  length_prop_value_p = ecma_create_named_data_property (obj_p,
                                                         length_magic_string_p,
                                                         ECMA_PROPERTY_FLAG_WRITABLE,
                                                         NULL);

(read-only should be "not writable" in javascript-speak).

[jimmy-huang]

@pfalcon thanks for pointing that out, but ecma_ are internal apis, not the public apis we can use, we should file a feature request to have a jerry_ api to allow setting the property

[grgustaf]

Fixed by PR #576. This makes test-buffer.js pass all 18/18 subtests.

[grgustaf]

BTW @jimmy-huang and @pfalcon, I found that this and more can be done using jerry_define_own_property.