Fix parsing ts type casts and nested patterns in destructuring #14500

nicolo-ribaudo · 2022-04-27T23:42:41Z

Q	A
Fixed Issues?	Fixes #14498, fixes #14504
Patch: Bug Fix?	Yes
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

nicolo-ribaudo · 2022-04-27T23:43:36Z

packages/babel-parser/src/parser/lval.js

@@ -625,7 +631,8 @@ export default class LValParser extends NodeUtils {

    const validity = this.isValidLVal(
      expression.type,
-      hasParenthesizedAncestor || expression.extra?.parenthesized,
+      !(hasParenthesizedAncestor || expression.extra?.parenthesized) &&
+        ancestor.type === "AssignmentExpression",


This code was needed to disallow a as b = c, however we must now make sure to not disallow [a as b] = c.

Can you add a new test case?

for (a as b of []);

It is disallowed in 7.16.2 but tsc allows it, and it seems this PR also fixes this one, too.

nicolo-ribaudo · 2022-04-27T23:44:40Z

...ges/babel-parser/test/fixtures/typescript/cast/unparenthesized-assert-and-assign/output.json

@@ -3,8 +3,7 @@
  "start":0,"end":46,"loc":{"start":{"line":1,"column":0,"index":0},"end":{"line":2,"column":29,"index":46}},
  "errors": [
    "SyntaxError: Invalid left-hand side in assignment expression. (1:0)",
-    "SyntaxError: Invalid left-hand side in assignment expression. (2:6)",
-    "SyntaxError: Invalid left-hand side in object destructuring pattern. (2:6)"
+    "SyntaxError: Invalid left-hand side in assignment expression. (2:6)"


These two errors were at the same location.

It would be better to only keep the "object destructuring pattern" rather than "assignment expression", but it's quite complex because it requires delaying errors until we know what the outer object is (destructuring or object). I might improve it in a follow-up PR.

babel-bot · 2022-04-27T23:45:13Z

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/51815/

JLHwung · 2022-04-28T00:36:02Z

packages/babel-parser/src/parser/lval.js

@@ -625,7 +631,8 @@ export default class LValParser extends NodeUtils {

    const validity = this.isValidLVal(
      expression.type,
-      hasParenthesizedAncestor || expression.extra?.parenthesized,
+      !(hasParenthesizedAncestor || expression.extra?.parenthesized) &&
+        ancestor.type === "AssignmentExpression",


Can you add a new test case?

for (a as b of []);

It is disallowed in 7.16.2 but tsc allows it, and it seems this PR also fixes this one, too.

I'm going to push a few more commits for the new comments in #14498

nicolo-ribaudo · 2022-04-28T21:46:19Z

@thorn0 I would appreciate if you could check if there is any missing new test!

thorn0 · 2022-04-28T22:50:45Z

({ ...[] }=x); and ({ ...{} }=x); should be errors, but they're parsed successfully if the typescript plugin is enabled. REPL

thorn0 · 2022-04-28T23:18:14Z

(a as T)! = b: TS, Babel (this PR)
Same with a!! = b.

nicolo-ribaudo · 2022-04-29T16:18:12Z

Thanks, fixed!

JLHwung · 2022-04-29T18:47:16Z

packages/babel-parser/src/parser/lval.js

@@ -162,11 +171,10 @@ export default class LValParser extends NodeUtils {
      }

      case "SpreadElement": {


We can handle SpreadElement in toAssignableObjectExpressionProp and toAssignableList, so we don't need to track isInObjectPattern anymore.

Done! 69719e4

nicolo-ribaudo · 2022-04-29T23:11:34Z

...-parser/test/fixtures/es2018/object-rest-spread/expression-rest-not-last-invalid/output.json

@@ -23,7 +22,7 @@
            "start":1,"end":13,"loc":{"start":{"line":1,"column":1,"index":1},"end":{"line":1,"column":13,"index":13}},
            "properties": [
              {
-                "type": "SpreadElement",
+                "type": "RestElement",


This is a slight improvement, since it gives a better shape to the recovered AST (the input code is ({ ...rest, x } = {}).

The function sometimes mutated its argument, sometimes it returned a new node. However, almost all usages didn't handle the "new node" case: this commit removes it, and the function always mutates its argument.

packages/babel-types/src/definitions/core.ts

nicolo-ribaudo added PR: Bug Fix 🐛 A type of pull request used for our changelog categories pkg: parser area: typescript labels Apr 27, 2022

nicolo-ribaudo commented Apr 27, 2022

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JLHwung previously approved these changes Apr 28, 2022

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existentialism previously approved these changes Apr 28, 2022

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thorn0 mentioned this pull request Apr 28, 2022

[Bug]: Weird AST for as in ArrayPattern #14498

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nicolo-ribaudo requested review from existentialism and JLHwung April 28, 2022 21:41

nicolo-ribaudo changed the title ~~[ts] Fix AST for type assertions in destructuring~~ Fix parsing for type assertions and nested patterns in destructuring Apr 28, 2022

nicolo-ribaudo changed the title ~~Fix parsing for type assertions and nested patterns in destructuring~~ Fix parsing of ts type casts and nested patterns in destruct Apr 28, 2022

JLHwung approved these changes Apr 29, 2022

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nicolo-ribaudo commented Apr 29, 2022

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nicolo-ribaudo force-pushed the ts-ast-as-in-destructuring branch from 4cbc10d to 69719e4 Compare April 29, 2022 23:17

JLHwung approved these changes Apr 30, 2022

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existentialism approved these changes Apr 30, 2022

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nicolo-ribaudo added 7 commits May 1, 2022 13:02

[ts] Fix AST for type assertions in destructuring

e7747b3

Add for-of test

408db9b

Always return void from toAssignable

35e9ee9

The function sometimes mutated its argument, sometimes it returned a new node. However, almost all usages didn't handle the "new node" case: this commit removes it, and the function always mutates its argument.

Fix handling of nested type casts

1de2335

Update comment

bd5e86d

Add test for babel#14504

f4f12aa

Fix flow

1dd0481

nicolo-ribaudo added 7 commits May 1, 2022 13:02

Disallow {...{}} also with flow/ts plugins

66f28e8

Support in @babel/types

f3402e9

Update generated files

bea42da

Remove isInObjectPattern param from toAssignable

0a2d3a3

Fix ts error

cdbd0a3

Fix typo

d86a2b5

Fix Babel 8

8be7c27

nicolo-ribaudo force-pushed the ts-ast-as-in-destructuring branch from 72cc585 to 8be7c27 Compare May 1, 2022 11:02

nicolo-ribaudo changed the title ~~Fix parsing of ts type casts and nested patterns in destruct~~ Fix parsing ts type casts and nested patterns in destructuring May 1, 2022

nicolo-ribaudo merged commit 0e6900d into babel:main May 1, 2022

nicolo-ribaudo deleted the ts-ast-as-in-destructuring branch May 1, 2022 12:39

JLHwung reviewed May 2, 2022

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packages/babel-types/src/definitions/core.ts Show resolved Hide resolved

HosokawaR mentioned this pull request Jun 12, 2022

Revert "print as instead of colon for babel-ts parser" prettier/prettier#12995

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github-actions bot added the outdated A closed issue/PR that is archived due to age. Recommended to make a new issue label Aug 2, 2022

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Fix parsing ts type casts and nested patterns in destructuring #14500

Fix parsing ts type casts and nested patterns in destructuring #14500

nicolo-ribaudo commented Apr 27, 2022 •

edited

nicolo-ribaudo Apr 27, 2022

JLHwung Apr 28, 2022

nicolo-ribaudo Apr 27, 2022

babel-bot commented Apr 27, 2022 •

edited

JLHwung Apr 28, 2022

nicolo-ribaudo commented Apr 28, 2022 •

edited

thorn0 commented Apr 28, 2022

thorn0 commented Apr 28, 2022 •

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nicolo-ribaudo commented Apr 29, 2022

JLHwung Apr 29, 2022

nicolo-ribaudo Apr 29, 2022

nicolo-ribaudo Apr 29, 2022

		@@ -162,11 +171,10 @@ export default class LValParser extends NodeUtils {
		}

		case "SpreadElement": {

Fix parsing ts type casts and nested patterns in destructuring #14500

Fix parsing ts type casts and nested patterns in destructuring #14500

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