Getting json body in aws Lambda via API gateway

Question

I\'m currently using NodeJS to build a bot on AWS lambda via AWS Api Gateway and I\'m running into an issue with POST requests and JSON data. My api uses \'Use Lambda Proxy

Answer 1

I think there are a few things to understand when working with API Gateway integration with Lambda.

Lambda Integration vs Lambda Proxy Integration

There used to be only Lambda Integration which requires mapping templates. I suppose this is why still seeing many examples using it.

• How to pass a querystring or route parameter to AWS Lambda from Amazon API Gateway

  As of September 2017, you no longer have to configure mappings to access the request body.

• Serverless Architecture on AWS

  Lambda Proxy Integration, If you enable it, API Gateway will map every request to JSON and pass it to Lambda as the event object. In the Lambda function you’ll be able to retrieve query string parameters, headers, stage variables, path parameters, request context, and the body from it.
  
  Without enabling Lambda Proxy Integration, you’ll have to create a mapping template in the Integration Request section of API Gateway and decide how to map the HTTP request to JSON yourself. And you’d likely have to create an Integration Response mapping if you were to pass information back to the client.
  
  Before Lambda Proxy Integration was added, users were forced to map requests and responses manually, which was a source of consternation, especially with more complex mappings.

body is escaped string, not JSON

Using Lambda Proxy Integration, the body in the event of lambda is a string escaped with backslash, not a JSON.

"body": "{\"foo\":\"bar\"}" 

If tested in a JSON formatter.

Parse error on line 1:
{\"foo\":\"bar\"}
-^
Expecting 'STRING', '}', got 'undefined'

The document below is about response but it should apply to request.

• I received a "malformed Lambda proxy response" error or a 502 status code with Amazon API Gateway. How do I resolve this?

  The body field, if you are returning JSON, must be converted to a string or it will cause further problems with the response. You can use JSON.stringify to handle this in Node.js functions; other runtimes will require different solutions, but the concept is the same.

For JavaScript to access it as a JSON object, need to convert it back into JSON object with json.parse in JapaScript, json.dumps in Python.

• How To Work with JSON in JavaScript

  Strings are useful for transporting but you’ll want to be able to convert them back to a JSON object on the client and/or the server side.

The AWS documentation shows what to do.

if (event.body !== null && event.body !== undefined) {
    let body = JSON.parse(event.body)
    if (body.time) 
        time = body.time;
}
...
var response = {
    statusCode: responseCode,
    headers: {
        "x-custom-header" : "my custom header value"
    },
    body: JSON.stringify(responseBody)
};
console.log("response: " + JSON.stringify(response))
callback(null, response);