Double error on invalid delete of readonly property

[jakebailey]

Bug Report

🔎 Search Terms

operand delete operator double diagnostic error

🕗 Version & Regression Information

• This changed between versions 3.9 and 4.0

Before 4.0, we only gave one error here. In 4.0 and above, we give two errors.

⏯ Playground Link

Playground Link

💻 Code

class Foo {}
delete Foo.name;

🙁 Actual behavior

Two errors

🙂 Expected behavior

One error

[ahmad-04]

Hi, I wanted to work on this where should I start.

[Bismay5467]

Hey ! I have possible explanation for the second error.

Error : The operand of a 'delete' operator must be optional.

Explanation : That was a false positive where the prototype chain still includes the property - bcoz the delete statement only works on direct properties, and not on inherited properties. This got corrected in the later versions of typescript.

One possible fix : declare 'name' member inside class as optional.

[jakebailey]

This bug doesn't seem to reproduce in a compiler or fourslash test, so this looks to be something in the services causing a duplicate error to occur (not uncommon). In particular, the The operand of a 'delete' operator cannot be a read-only property. message does not show up in testing.

@ahmad2901 Likely, this needs a test like in #52696 or #52373 which baselines the old bad behavior, followed by a change which fixes the bug and changes the baseline to only contain one error.

@Bismay5467 I don't think that's related, given it doesn't reproduce in a compiler test, only in the editor, and this bug is about that inconsistency.

[ahmad-04]

Hey, @jakebailey I am new to typescript can you recommend where should I get started?

[jakebailey]

The place to start would be to find all places in the checker which emit this error message, set a breakpoint at each, and figure out in which conditions both appear, and see which one shouldn't be happening.

[ahmad-04]

sure thanks for your help.

[ahmad-04]

@jakebailey by checker do you mean tests folder??

[somebody1234]

@jakebailey i can't repro on 5.0.2 and 4.0 - i get the second error even when running tsc, unless that's not what compiler and fourslash tests do? note that checkDeleteExpressionMustBeOptional explicitly checks for strictNullChecks so it will not be emitted if that's off

the fix looks like it'd be simple to me so i'm likely just misunderstanding something here

            if (isReadonlySymbol(symbol)) {
                error(expr, Diagnostics.The_operand_of_a_delete_operator_cannot_be_a_read_only_property);
                // missing return, so tsc *may* emit a second error from the below function call
            }
            checkDeleteExpressionMustBeOptional(expr, symbol);

[shoeb00]

Hey @jakebailey , any updates on this issue? I am unable to replicate this issue. I have tried versions 3.0, 3.9, 4.0 and 4.9 and there was a single error for all of them

test.ts:2:8 - error TS2704: The operand of a 'delete' operator cannot be a read-only property.

2 delete Foo.name;
~~~~~~~~

Found 1 error in test.ts:2

[jakebailey]

I don't have any updates. This only reproduces in the language service and the playground, as I said before. If you check the latter by the link in my issue, you'll see it says there are two errors on the right.

[shoeb00]

This is my first time contributing to an open-source project, I would really appreciate your help. I have cloned the repo and switched to the release-4.0 branch then I ran these commands

npm install && npm run build

Then I created a test.ts file with the code mentioned in the issue then to run that file I used this command

node built/local/tsc.js test.ts

Output:

test.ts:2:8 - error TS2704: The operand of a 'delete' operator cannot be a read-only property.

2 delete Foo.name;


Found 1 error in test.ts:2

I am actively looking for issues where I can contribute, please feel free to recommend any other issues/bugs or study material that can help me

[gooddavvy]

What are the errors? Can you be more specific?

[somebody1234]

@gooddavvy check the "Playground Link" heading in the original post

[onsevenine]

I think to it's trying to get an operand which could be empty, a property which has no use but still exist in the object.

[SauravChittal]

Is this issue fixed, or could I work on it?

[jakebailey]

There is #53961 which presumably fixes the issue but isn't yet complete.

Also note: https://github.com/microsoft/TypeScript/blob/main/CONTRIBUTING.md#issue-claiming