How to get optional chaining working in TypeScript?

问题

Looks like optional chaining has landed. Here's an example

What I can't figure out is how to get TS to compile it properly. I'm not getting any syntax errors in my project, but this:

let imageFileId = (await db.query(sql`select id from image_files where sha256=${sha256}`))[0]?.id;

Is being output as:

let imageFileId = (await db.query(mysql3_1.sql `select id from image_files where sha256=${sha256}`))[0]?.id;

Which won't run until we get native support in Node.

Here's my tsconfig:

{
    "compilerOptions": {
        "strict": true,
        "importHelpers": false,
        "inlineSources": true,
        "noEmitOnError": true,
        "pretty": true,
        "module": "commonjs",
        "noImplicitAny": true,
        "suppressImplicitAnyIndexErrors": false,
        "removeComments": false,
        "preserveConstEnums": false,
        "sourceMap": true,
        "lib": ["es2018"],
        "skipLibCheck": false,
        "outDir": "dist",
        "target": "esnext",
        "declaration": false,
        "resolveJsonModule": true,
        "esModuleInterop": false,
        "moduleResolution": "node",
        "allowSyntheticDefaultImports": true,
        "baseUrl": ".",
        "paths": {
            "*": ["src/*"]
        },
        "noEmit": false
    },
    "files": [
        "src/index"
    ],
    "include": [
        "src/**/*.d.ts"
    ]
}

Is there some other option I need to enable to compile the ?. operator?

Please note I'm not using Babel and I don't want to bring it into the picture.

回答1:

The problem is you are targeting esnext this will tell the compiler to output all language features as is without any transpilation. Set the language to es2020 (or below) and ?. and ?? will get transpiled to compatible code:

(async function () {
    let imageFileId = (await db.query(sql`select id from image_files where sha256=${sha256}`))[0]?.id;
})()

Playground Link

There is no fine-grained control over which language features get transpiled and which don't do you have to pick a version as a whole unfortunately,

回答2:

Well, I didn't want to use Babel because then I'd have to figure out how to replace ts-node. There's a bunch of outdated docs out there referring to old Babel packages, but these instructions should work as of Nov 2019:

Add a .babelrc file:

{
    "presets": [
        ["@babel/preset-env",{"targets": {"node": "current"}}],
        "@babel/preset-typescript"
    ],
    "plugins": [
        "@babel/plugin-syntax-bigint"
    ]
}

Add these deps:

  "devDependencies": {
    "@babel/cli": "^7.7.0",
    "@babel/core": "^7.7.0",
    "@babel/node": "^7.7.0",
    "@babel/plugin-syntax-bigint": "^7.4.4",
    "@babel/preset-env": "^7.7.1",
    "@babel/preset-typescript": "^7.7.0",
    "@types/node": "^12.7.5",
    "typescript": "^3.7.2"
  }

Execute your code with:

node_modules/.bin/babel-node --extensions ".ts" src/index.ts

The --extensions ".ts" is very important, even though you're explicitly trying to execute a .ts file, it won't transpile it w/out that.

I like to use GNU Make instead of package.json scripts:

MAKEFLAGS += --no-builtin-rules
.SUFFIXES:
NM := node_modules/.bin
.PHONY: build start dev clean test publish

## commands
########################################

__default:
    $(error Please specify a target)

build: build-types build-js dist/package.json

build-types: node_modules/.yarn-integrity
    $(NM)/tsc --emitDeclarationOnly

build-js: node_modules/.yarn-integrity
    $(NM)/babel src --out-dir dist --extensions ".ts" --source-maps inline

run: node_modules/.yarn-integrity
    $(NM)/babel-node --extensions ".ts" src/index.ts

check: node_modules/.yarn-integrity
    $(NM)/tsc --noEmit

dist:
    mkdir -p $@

clean:
    rm -rf node_modules dist yarn-error.log

dist/package.json: package.json | dist
    jq 'del(.private, .devDependencies, .scripts, .eslintConfig, .babel)' $< > $@

## files
########################################

node_modules/.yarn-integrity: yarn.lock
    @yarn install --frozen-lockfile --production=false --check-files
    @touch -mr $@ $<

yarn.lock: package.json
    @yarn check --integrity
    @touch -mr $@ $<

Or just copy from Microsoft's TypeScript Babel Starter.

来源：https://stackoverflow.com/questions/58725711/how-to-get-optional-chaining-working-in-typescript