Situation When we are using NodeJS as the runtime for AWS Lambdas, JavaScript is the default language. However, due to the lack of typing check in JavaScript, from time to time, buggy codes are deployed to Lambda inadvertently. Such as a small typo like this: exports.lambdaHandler = (event, context) => { queries = event.queytStringParameters; } async const // ... We mean to get que StringParameters, but the queries ends up undefined as a result of carelessness. ry Goal We want to utilize TypeScript to write lambda handlers. With TypeScript, we will have the following benefits: Vanilla code completion hints while programmingCompilation time error checks to avoid redundant deployments It is not hard to do so, this article will introduce 3 steps to complete that job. Preparation Before going through the 3 steps, let’s create a classic lambda project with : SAM CLI sam init After the above command, we will get a folder with these files: ├── README.md
├── events
│   └── event.json
├── hello-world
│   ├── app.js
│   ├── package.json
│   └── tests
│       └── unit
│           └── test-handler.js
└── template.yaml Then we will start shift this JS package into a TS package. Step 1: Add TypeScript Dependency In the package.json, add the following codes: : { : }, : { : , : , : , : } "scripts" "compile" "tsc" "devDependencies" "aws-sdk" "^2.655.0" "@types/aws-lambda" "^8.10.51" "@types/node" "^13.13.5" "typescript" "^3.8.3" : this will be used to compile the TypeScript code to JavaScript scripts/compile : since this is only for development, we do not need to add the packages to the dependencies block devDependencies : depending on whether you are using AWS SDK in your lambda aws-sdk : this is very vital for the code completion and typing checks @types/aws-lambda : we need this package for built-in types @types/node : where the tsc is from typescript Step 2: Add tsconfig.json { : { : , : , : , : , : , : }, : [ ], : [ , ]
} "compilerOptions" "module" "CommonJS" "target" "ES2017" "noImplicitAny" true "preserveConstEnums" true "outDir" "./built" "sourceMap" true "include" "src-ts/**/*" "exclude" "node_modules" "**/*.spec.ts" TypeScript compiler needs tsconfig.json to figure out how to transform TypeScript to JavaScript. : is fine here module CommonJS : applying will keep the async and await syntax instead of transforming them to Promise code. Since we are using Node12 as the runtime environment, Lambda function supports interpreting the syntax. Meanwhile, keeping async and await makes code clean and short target ES2017 : recommended to have. Compiler will throw an error if there is a variable declared without type noImplicitAny : more like a syntax sugar, but I would like to keep it on because it could keep enum classes in the JavaScript code in the form of object, which helps understand the JavaScript code preserveConstEnums : any folder you would like to set as the compilation output outDir : this one is optional sourceMap Step 3: Change the code First, create the folder src-ts, and move app.js to that folder. The app.js looks like this now: exports.lambdaHandler = (event, context) => { queries = .stringify(event.queytStringParameters); { : , : }
}; async const JSON return statusCode 200 body `Queries: ` ${queries} Let’s create app.ts and replace it: { 
  APIGatewayProxyEvent, 
  APIGatewayProxyResult 
} ; lambdaHandler = (
  event: APIGatewayProxyEvent
): <APIGatewayProxyResult> => { queries = .stringify(event.queryStringParameters); {
    statusCode: ,
    body: }
} import from "aws-lambda" export const async Promise const JSON return 200 `Queries: ` ${queries} Since this TypeScript file will be compiled to built folder, we also need to modify the Handler field in template.yaml, to ensure the lambda resource could locate the code in the right path: Resources: HelloWorldFunction: Type: AWS::Serverless::Function Properties: CodeUri: hello-world/built Handler: app.lambdaHandler We append the path built by the folder hello-world, so that AWS Lambda could find the handler correctly. Now the directory looks like below: ├── README.md
├── hello-world
│   ├── built
│   │   ├── app.js
│   │   └── app.js.map
│   ├── package-lock.json
│   ├── package.json
│   ├── src-ts
│   │   ├── app.ts
│   │   └── tests
│   └── tsconfig.json
├── samconfig.toml
└── template.yaml Deploy and Test hello-world
npm install
npm run compile ..
sam deploy --guided cd cd After deployed successfully, we will see the Lambda function in AWS Console as follows: And we should be able to test the Lambda function using the following command: ▶ curl https://[API_ID].amazonaws.com/Prod/hello\?weather\=sunny
Queries: { : } "weather" "sunny" Conclusion and Next Step I find it really refreshing after utilizing TypeScript support for my Lambda functions. On one hand, it could save me tons of time looking up the API Doc for a method name or the parameter list. On the other hand, tsc also helps me to detect any potential problem before deploying. In the next article, I will talk about how to do local integration tests on Lambda functions.

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