[AST] shape of Param, Pat and VarDeclarator

[magic-akari]

Describe the bug

I don't think the typeAnnotation should be attached on pat.

1. VarDeclarator

The typeAnnotation should always be attached on the VarDeclarator, not the pat.

const a: number = 1, [b]: [number] = 2, { c }: { c: number } = { c: 3 };

Expect:

VarDeclarator {
    name: Pat,
    init: Expr,
    type_ann: TsTypeAnn,
};

Actual:
the type_ann is in the name.

2. Param

There should be a type_ann in the Param like VarDeclarator.

3. init in the Param

function foo(a = 1) { }

function bar(a: number = 1) { }

Expect :

Param {
    pat: Pat,
    init: Expr,
    type_ann: TsTypeAnn
}

Actual:
got AssignPat

---

It's better to remove typeAnnotation from Pat and add it to other AST struct like Param, VarDeclarator.

Input code

No response

Config

No response

Playground link

No response

Expected behavior

/

Actual behavior

No response

Version

N/A

Additional context

No response

[kdy1]

Not sure if it will work.
AST of swc is designed after one of the babel

[magic-akari]

Yes, I checked the Babel AST.

function foo(a = 1) { }

It is parsed as AssignmentPattern as well.

In TypeScript, however, there is an initializer field in the Parameter.

Rome is also designed in this way.

I cannot tell which is better.

[magic-akari]

After some investigating, I found out that babel's AST design is related to flow.js.
In older versions of flow.js(<v0.176.0), it was indeed possible to write the type into the pattern.

const [a: number, b: string, c: boolean] = ['a', true, 1];
a.toLocaleLowerCase(); // a === string
b - 1 // b === boolean
c.toFixed(); // c === number

const arr: string[] = [];
const [d: number] = arr;
d.toLocaleLowerCase();

https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVBjOA7AzgFzAG0BDALjGwFcBbAIwFMAnAGjDooKYEtsBzNhgp04cGAxLYAumAC8xAOQkFbfEyoM2ARikBuVCQB0+OABk4GEuPMJmAYRK4GACgCUusMGBgSc2fK5ePlQ6MABaMC1Pb1D-eRExCWxMYzgAMW4ADwYAEzcPLzAMP3lqemZ0LDxCEiYmTjUgohl5Zv0qgmIcijLGJhafOv0c1PNLa0R7Rxd3VCA
(from babel/babel#9545 (comment))

But in the new version of flow.js(>=0.176.0), it has behaved like TS and does not allow this syntax anymore.

Babel is also trying to fix it.

• Design details of the Babel AST babel/babel#14834 (comment)
• [breaking] Make Identifier an "atomic" node babel/babel#9545