See this example on TypeScript Playground:

const obj: {
    outer?: {
        inner?: string
    }
} = {};

console.log(obj?.outer?.inner);

Playground Link

Even when targeting ES2017, the optional chaining operator is compiled to basic JS code which all current browsers understand.

So no need to worry :)

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 There's a bunch of outdated docs out there referring to old Babel packages, but these instructions should work as of Nov 2019: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:

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

Add these deps:

Copy  "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:

Copynode_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:

CopyMAKEFLAGS += --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.