@aws-sdk/types 3.10.0 broke Typescript 3 compatibility

[legraphista]

Describe the bug

This line is not backward compatible to Typescript V3

aws-sdk-js-v3/packages/types/src/util.ts

Line 109 in 4a33342

export type UserAgentPair = [name: string, version?: string];

Your environment

SDK version number

@aws-sdk/[email protected]

Is the issue in the browser/Node.js/ReactNative?

Node.js

Details of the browser/Node.js/ReactNative version

node -v: v12.22.1
tsc -v: Version 3.9.9

Steps to reproduce

package.json

{
  "dependencies": {
    "@aws-sdk/types": "^3.10.0"
  }
}

test.ts

import '@aws-sdk/types'

terminal

yarn && tsc --lib test.ts

Observed behavior

node_modules/@aws-sdk/types/dist/types/util.d.ts:92:42 - error TS1005: ',' expected.

92 export declare type UserAgentPair = [name: string, version?: string];
                                            ~

node_modules/@aws-sdk/types/dist/types/util.d.ts:92:60 - error TS1005: ',' expected.

92 export declare type UserAgentPair = [name: string, version?: string];
                                                              ~

Expected behavior

test.ts should compile with no errors

Additional context

@aws-sdk/types v3.6.1 had no issues compiling in TS3

[trivikr]

This issue seems to be happening, as postbuild is run twice on packages/types

Verified as follows:

> yarn build
yarn run v1.22.5
$ yarn build:es && yarn build:cjs
$ tsc -p tsconfig.es.json
$ tsc -p tsconfig.cjs.json
$ downlevel-dts dist/types dist/types/ts3.4
Done in 9.06s.

> ls dist/types 
abort.d.ts  client.d.ts  command.d.ts  credentials.d.ts  crypto.d.ts  eventStream.d.ts  http.d.ts  index.d.ts  logger.d.ts  middleware.d.ts  pagination.d.ts  response.d.ts  serde.d.ts  signature.d.ts  transfer.d.ts  ts3.4  util.d.ts

> ls dist/types/ts3.4 
abort.d.ts  client.d.ts  command.d.ts  credentials.d.ts  crypto.d.ts  eventStream.d.ts  http.d.ts  index.d.ts  logger.d.ts  middleware.d.ts  pagination.d.ts  response.d.ts  serde.d.ts  signature.d.ts  transfer.d.ts  util.d.ts

> yarn build
yarn run v1.22.5

> yarn build:es && yarn build:cjs
$ tsc -p tsconfig.es.json
$ tsc -p tsconfig.cjs.json
$ downlevel-dts dist/types dist/types/ts3.4
Done in 4.33s.
dev-dsk-trivikr-2b-b5d0913a: types> ls dist/types/ts3.4
abort.d.ts  client.d.ts  command.d.ts  credentials.d.ts  crypto.d.ts  eventStream.d.ts  http.d.ts  index.d.ts  logger.d.ts  middleware.d.ts  pagination.d.ts  response.d.ts  serde.d.ts  signature.d.ts  transfer.d.ts  ts3.4  util.d.ts

This could be happening as packages/types is built twice, once as part of crypto-dependencies and second time as part of lerna run build.

aws-sdk-js-v3/package.json

Line 18 in 4a33342

"build:all": "yarn build:crypto-dependencies && lerna run build",

[trivikr]

A probable fix would be to move downlevel-dts from postbuild to prepublishOnly for the following reasons:

• The CI hasn't failed because of backward incompatible types added in feat: use downlevel-dts to generate TS 3.4 compatible types #1943
• The build takes lot of time since the introduction of downlevel-dts in postbuild, and developers usually don't test with old versions of TypeScript.

A GitHub Action needs to be written to regularly run prepublish to ensure types are backward compatible.

[trivikr]

Metrics:

build time with downlevel-dts in postbuild

$ git clean -dfx && yarn

$ time yarn build:all
...
Done in 850.77s.
yarn build:all  12588.02s user 307.90s system 1515% cpu 14:10.90 total

build time with downlevel-dts in prepublishOnly

$ git clean -dfx && yarn

$ time yarn build:all
...
Done in 736.90s.
yarn build:all  10956.08s user 248.51s system 1520% cpu 12:17.03 total

The SDK maintainers can explicitly run prepublishOnly if they want to test downlevel-dts

$ ./node_modules/.bin/lerna run prepublishOnly

[github-actions]

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