Provide Grammar Errors for JavaScript Files

[DanielRosenwasser]

Recently, we started reporting high-confidence errors for typos as suggestions in the language service. We believe that there is a set of grammar issues that are worth erroring on as well that are not reported today (e.g. duplicate block-scoped variables, extending multiple classes, rest parameters occurring before regular parameters, etc.)

Open questions:

• How do we implement this as part of our API?
• What sorts of errors are acceptable here?
• Would we be giving errors that run against the expectations of any popular runtimes, build tools, or syntax extensions?

[sandersn]

Here are my rough notes, cleaned up a little. This is only the binder errors.

Runtime Error

These errors are all errors in JS, at least in V8. Mostly they only apply in strict contexts: .mjs files and class bodies. Actually, the errors are good enough that they should use the same rule as for TS: any JS file that uses ES exports counts as a real ES module and is in strict mode. (Even though .js files are usually commonjs at present.)

These errors should always be provided to JS files.

Diagnostics.Cannot_redeclare_block_scoped_variable_0

const x = 1
var x = 1

Diagnostics.A_module_cannot_have_multiple_default_exports

also:

• Diagnostics.A_module_cannot_have_multiple_default_exports
• Diagnostics.Another_export_default_is_here
• Diagnostics.and_here <-- (!) this might be used elsewhere
• Diagnostics.The_first_export_default_is_here

export default 12;
export default 13;

Diagnostics.Identifier_expected_0_is_a_reserved_word_at_the_top_level_of_a_module

export default 12;
const await = 1

Diagnostics.Identifier_expected_0_is_a_reserved_word_in_strict_mode_Modules_are_automatically_in_strict_mode

const yield = 2
export default 12;

Diagnostics.Identifier_expected_0_is_a_reserved_word_that_cannot_be_used_here

async function f() {
  const await = 1
}

Diagnostics.Identifier_expected_0_is_a_reserved_word_that_cannot_be_used_here

function* f() {
  const yield = 1
}

Diagnostics.constructor_is_a_reserved_word

class C {
    #constructor = 1
}

Diagnostics.delete_cannot_be_called_on_an_identifier_in_strict_mode

class C {
    m() {
        function container(f) {
            delete f
        }
        var g = 1
        delete g
        delete container
    }
}

Diagnostics.Invalid_use_of_0_Class_definitions_are_automatically_in_strict_mode

class C {
    m() {
        const eval = 1
        const arguments = 2
    }
    g() {
    }
}

Diagnostics.Invalid_use_of_0_Modules_are_automatically_in_strict_mode

export default 13;
const eval = 1
const arguments = 2

Diagnostics.Invalid_use_of_0_in_strict_mode

"use strict" // this doesn't actually turn strict on or off
const eval = 1
const arguments = 2

Diagnostics.Octal_literals_are_not_allowed_in_strict_mode

class C {
    m() {
      return 010
    }
}

Diagnostics.with_statements_are_not_allowed_in_strict_mode

class C {
    m() {
      const n = 010
      with (n) {
        return toFixed()
      }
    }
}

Likely wrong

These errors indicate code that is likely wrong, but won't error.

I'm not sure whether to show these errors in plain JS files.

• Diagnostics.An_object_literal_cannot_have_multiple_properties_with_the_same_name_in_strict_mode

class C {
  m() {
    return { a: 1, a: 1 }
  }
}

(I'm not sure what 'strict mode' refers to here; the above test works fine in node for .js and .mjs)
From a JS perspective, this feels linty, so I don't think I'll include it.

• Diagnostics.A_label_is_not_allowed_here

    class C {
        m() {
            label: var x = 1
            break label
        }
    }

There are two reasons I can see for this error:

1. Intended object literal -- although it's only issued before declarations like var, function, etc, which are not valid property names.
2. You misunderstand the intent of labels, and try to use the label later.

I still think (1) is possible because there are surely some declarations that start with contextual keywords. And our error for (2) is abysmally misleading, even though when you run such a program, you'll get a clear runtime error. So I'm going to include this error, even though I'm going to skip the "Unused label" that applies to valid label locations.

Harmless/Could be intended

These errors indicate code that could be written by reasonable people trapped by circumstance into being careless.

Probably shouldn't show these errors in JS.

• Diagnostics.Duplicate_identifier_0
• Diagnostics.Unused_label
• Diagnostics.Identifier_expected_0_is_a_reserved_word_in_strict_mode_Class_definitions_are_automatically_in_strict_mode
• Diagnostics.Identifier_expected_0_is_a_reserved_word_in_strict_mode_Modules_are_automatically_in_strict_mode
• Diagnostics.Identifier_expected_0_is_a_reserved_word_in_strict_mode <-- (probably)
• Diagnostics.Unreachable_code_detected

Sir Not-Appearing-in-this-Language

These errors only apply to Typescript constructs or Typescript emit.

• Diagnostics.Enum_declarations_can_only_merge_with_namespace_or_other_enum_declarations
• Diagnostics.Did_you_mean_0
• Diagnostics.export_modifier_cannot_be_applied_to_ambient_modules_and_module_augmentations_since_they_are_always_visible
• Diagnostics.Pattern_0_can_have_at_most_one_Asterisk_character
• Diagnostics.Function_declarations_are_not_allowed_inside_blocks_in_strict_mode_when_targeting_ES3_or_ES5_Class_definitions_are_automatically_in_strict_mode
• Diagnostics.Function_declarations_are_not_allowed_inside_blocks_in_strict_mode_when_targeting_ES3_or_ES5_Modules_are_automatically_in_strict_mode
• Diagnostics.Function_declarations_are_not_allowed_inside_blocks_in_strict_mode_when_targeting_ES3_or_ES5
• Diagnostics.Modifiers_cannot_appear_here
• Diagnostics.Global_module_exports_may_only_appear_at_top_level
• Diagnostics.Global_module_exports_may_only_appear_in_module_files
• Diagnostics.Global_module_exports_may_only_appear_in_declaration_files
• Diagnostics.Duplicate_identifier_0

[sandersn]

@orta points out that it would be extremely useful to show

Diagnostics.Cannot_assign_to_0_because_it_is_a_constant

in JS when you try to += 'foo' to a const string, for example.

[DanielRosenwasser]

In the same vein, used-before-defined would also be helpful for let``/const variables

[sandersn]

Here are all the relevant errors from the checker that are called from grammarErrorOnNode. This is not all the grammar errors, but it's a convenient way to bucket them. The checker errors seem a lot easier to distinguish between relevant and irrelevant, but maybe that's because I've looked at enough errors in a row now. There are 67 errors on this list, out of 184 grammarErrorOnNode calls.

• Diagnostics.Private_identifiers_are_not_allowed_outside_class_bodies
• Diagnostics.Private_identifiers_are_only_allowed_in_class_bodies_and_may_only_be_used_as_part_of_a_class_member_declaration_property_access_or_on_the_left_hand_side_of_an_in_expression
• Diagnostics.Cannot_find_name_0

class C {
    q = #unbound
    m() {
        #p++
        q++
        this.#q
        if (#po in this) {

        }
        const o = { a: 1, a: 2 }
        return o.a
    }
    #m() {
         this.#m = () => {}
    }
}
#unrelated

These 3 errors need improvement, but at least the first two should behave the same in JS as TS

• Diagnostics.Cannot_assign_to_private_method_0_Private_methods_are_not_writable

class C {
    #m() {
         this.#m = () => {}
    }
}

• Diagnostics.Private_identifiers_are_not_allowed_outside_class_bodies
  junk.#m (you get a better error if C is defined and even better if C.#m is defined)

• Diagnostics._0_and_1_operations_cannot_be_mixed_without_parentheses

var x = 1 || 2 ?? 2
var x = 2 ?? 3 || 4

• Diagnostics.For_await_loops_cannot_be_used_inside_a_class_static_block

class C {
    static {
        for await (const x of [1,2,3]) {
            console.log(x)
        }
    }
}

• Diagnostics.A_default_clause_cannot_appear_more_than_once_in_a_switch_statement

var b = true
switch (b) {
    case false:
        console.log('no')
    default:
        console.log('yes')
    default:
        console.log('wat')
}

• Diagnostics.Duplicate_label_0

label: for (const x in [1,2,3]) {
    label: for (const y in [1,2,3]) {
        break label;
    }
}

• Diagnostics.Cannot_redeclare_identifier_0_in_catch_clause

try {
    throw 2
}
catch (e) {
    const e = 1
    console.log(e)
}

• Diagnostics.Class_decorators_can_t_be_used_with_static_private_identifier_Consider_removing_the_experimental_decorator

declare var deco: any
@deco
class C {
    static #p = 1
}

But this doesn't run on any JS runtime today so it probably shouldn't apply

• Diagnostics.A_class_member_cannot_have_the_0_keyword

 class C {
    const x = 1
    const y() {
        return 12
    }
}

• Diagnostics._0_modifier_already_seen

class C {
    static static x = 1
}

• Diagnostics._0_modifier_must_precede_1_modifier

class C {
    static async x (){ }
}

• Diagnostics._0_modifier_cannot_appear_on_a_module_or_namespace_element

export static var x = 1

• Diagnostics._0_modifier_cannot_appear_on_a_parameter

function f(static x = 1) { return x }

• Diagnostics._0_modifier_already_seen

export export var x = 1

• Diagnostics._0_modifier_must_precede_1_modifier

async export function f(x = 1) { return x }

• Diagnostics._0_modifier_cannot_appear_on_class_elements_of_this_kind

class C {
    export p = 1
    export m() {
    }
}

Note that there's an existing, bogus error in JS which is supposed to be issued on
exports from namespaces, which are not in JS of course.

• Diagnostics._0_modifier_cannot_appear_on_a_parameter

function f(export x = 1) { return x }

• Diagnostics._0_modifier_must_precede_1_modifier

default export 13;

• Diagnostics._0_modifier_already_seen

async async function f() {
    return 1
}

• Diagnostics._0_modifier_cannot_appear_on_a_parameter

function f(async x = 1) { return x }

• Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration

class C {
    static constructor() { }
}

• Diagnostics._0_modifier_cannot_appear_on_a_constructor_declaration

class C {
    async constructor() { }
}

• Diagnostics._0_modifier_cannot_be_used_here

async class C {
    async x = 1
}
async const y = 2
async import './other'
async export { C }

• Diagnostics.A_rest_parameter_must_be_last_in_a_parameter_list

function f (...x, y) {
}

• Diagnostics.A_rest_parameter_cannot_have_an_initializer

function f (...x = [1,2,3]) {
}

• Diagnostics.Line_terminator_not_permitted_before_arrow

const arr = x
  => x + 1

• Diagnostics.Tagged_template_expressions_are_not_permitted_in_an_optional_chain

var a = [1,2]
a?.`length`;

• Diagnostics.A_comma_expression_is_not_allowed_in_a_computed_property_name

const o = {
    [console.log('oh no'),2]: 'hi'
}

• InvalidQuestionMark

const o = {
    x?: 12
}

But: this already errors with a JS-specific error, so maybe the first should too.

const o = {
    m?() { return 12 }
}

• InvalidExclamationToken

var x = 1
const o = {
    x!
}
const o = {
    m!() { return 12 }
}

• Diagnostics.A_rest_element_cannot_contain_a_binding_pattern

const o = {
    x: 1, y: 2
}
let x; let y
;({ ...[] } = o)

• Diagnostics.Did_you_mean_to_use_a_Colon_An_can_only_follow_a_property_name_when_the_containing_object_literal_is_part_of_a_destructuring_pattern

const o = { x = 1 }

• Diagnostics.Private_identifiers_are_not_allowed_outside_class_bodies

const o = { #x: 1 }

• Diagnostics._0_modifier_cannot_be_used_here

const o = { export x: 1 }

• Diagnostics.Duplicate_identifier_0

TODO: Duplicated with An object literal cannot have multiple properties with the same name in strict mode.

const o = { 
    x: 1,
    x() { }

• Diagnostics.JSX_elements_cannot_have_multiple_attributes_with_the_same_name

• Diagnostics.JSX_attributes_must_only_be_assigned_a_non_empty_expression

• Diagnostics.JSX_property_access_expressions_cannot_include_JSX_namespace_names

• Diagnostics.JSX_expressions_may_not_use_the_comma_operator_Did_you_mean_to_write_an_array
  -- TODO Come back here and figure out what JSX errors are good --

• Diagnostics.The_left_hand_side_of_a_for_of_statement_may_not_be_async

let async
export const l = [1,2,3]
for (async of l) {
    console.log(x)
}

• Diagnostics.The_variable_declaration_of_a_for_in_statement_cannot_have_an_initializer

for (const x = 1 in [1,2,3]) {
    console.log(x)
}

• Diagnostics.The_variable_declaration_of_a_for_of_statement_cannot_have_an_initializer;

for (const x = 1 of [1,2,3]) {
    console.log(x)
}

• Diagnostics._0_Expected

class C {
    get x();
}

• Diagnostics.A_get_accessor_cannot_have_parameters

class C {
    get x(n) { return 1 }
}

• Diagnostics.A_set_accessor_must_have_exactly_one_parameter

class C {
    set x() { }
    set y(a, b) { }
}

• Diagnostics.A_set_accessor_cannot_have_rest_parameter

class C {
    set x(...n) { }
}

• Diagnostics.Jump_target_cannot_cross_function_boundary

function outer() {
    outer: for(;;) {
        function test() {
            break outer
        }
        test()
    }
}
outer()

• Diagnostics.A_continue_statement_can_only_jump_to_a_label_of_an_enclosing_iteration_statement

function outer(x) {
    outer: switch (x) {
        case 1:
            continue outer
    }
}
outer(1)

• Diagnostics.A_break_statement_can_only_jump_to_a_label_of_an_enclosing_statement

function outer(x) {
    break outer
}

• Diagnostics.A_continue_statement_can_only_jump_to_a_label_of_an_enclosing_iteration_statement;

function outer(x) {
    continue outer
}
outer(1)

• Diagnostics.A_break_statement_can_only_be_used_within_an_enclosing_iteration_or_switch_statement

function outer(x) {
    break
}

• Diagnostics.A_continue_statement_can_only_be_used_within_an_enclosing_iteration_statement;

function outer(x) {
    continue
}

• Diagnostics.A_rest_element_must_be_last_in_a_destructuring_pattern

const o = { e: 1, m: 1, name: "knee-deep" }
const { ...rest, e: a, m: n } = o
a + n

• Diagnostics.A_rest_element_cannot_have_a_property_name

const o = { e: 1, m: 1, name: "knee-deep" }
const { e: a, m: n, ...est: x } = o
a + n

• Diagnostics.A_destructuring_declaration_must_have_an_initializer

const { e: a, m: n }

• Diagnostics.const_declarations_must_be_initialized

const a

• Diagnostics.let_is_not_allowed_to_be_used_as_a_name_in_let_or_const_declarations

let let = 12

• Diagnostics.let_declarations_can_only_be_declared_inside_a_block

if (true)
  let c3 = 0

• Diagnostics.const_declarations_can_only_be_declared_inside_a_block

if (true)
  const c3 = 0

• Diagnostics._0_is_not_a_valid_meta_property_for_keyword_1_Did_you_mean_2

export let x = import.metal

• Diagnostics._0_is_not_a_valid_meta_property_for_keyword_1_Did_you_mean_2

function foo() { new.targe }

• Diagnostics.Classes_may_not_have_a_field_named_constructor

class C {
    "constructor" = 1
}

• Diagnostics.Dynamic_imports_can_only_accept_a_module_specifier_and_an_optional_assertion_as_arguments

const x = import()
const y = import('1', '2', '3')

• Diagnostics.Argument_of_dynamic_import_cannot_be_spread_element

const x = import(...[])