How can I parse event parametet in lambda?

Question

I am using Node.js 8.10 in lambda proxy integration. My goal is very simple.

send json data to lambda function
query rds with json data and retrive data from rds
return response with json data in lambda from rds data.

but I faced issue in step 1. I tried to figure out and I asked aws support center. it was not helpful. please help me.

my test json data are :

{
  "key1": "value1",
  "key2": "value2",
  "key3": "value3"
}

my code is very simple but have a problem :

exports.handler = async (event) => {
    var body = JSON.parse(event)//<--this is a problem

    let responseBody = {
        message: "hello",
        key1: body.key1
    };
    const response = {
        statusCode: 200,
        headers : {"Access-Control-Allow-Origin" : "*"},
        body: JSON.stringify(responseBody)
    };
    return response;
};

I got this error in second line.

{"errorMessage":"Unexpected token o in JSON at position 1","errorType":"SyntaxError","stackTrace":["JSON.parse (<anonymous>)","exports.handler (/var/task/index.js:2:21)"]}

So I changed second line code like this.

var body = JSON.parse(JSON.stringify(event))

and then I got response well. I thought that's working well.

{
  "statusCode": 200,
  "headers": {
    "Access-Control-Allow-Origin": "*"
  },
  "body": "{\"message\":\"hello\",\"key1\":\"value1\"}"
}

but I sent same json data with postman for real. but I got only this.

{ "message": "hello" }

there isn't "key1". so I added console.log and then I checked log in cloudwatch.

{ message: 'hello', key1: undefined }

I can see aws editor log is well but when I send data with postman. lambda couldn't parse my json data. it means lambda couldn't parse event parameter.

my questions are : 1. when I tried to parse like this

var body = JSON.parse(event)"

why does lambda editor issue error? Is there something wrong? this is super simple and very common code.

How can I parse my json data and return correct value not undefined. I expected this response

{ message: 'hello', key1: 'value1' }

I have been trying to solve this for 3days. but I really have no idea. please help me out.

It appears to me that the event parameter is not a JSON string as you expect. Rather it is a JavaScript object with several keys, including "statusCude", "headers" and "body". You probably need to look at event.body for the JSON you are expecting. — Code-Apprentice
– Code-Apprentice, Commented Mar 4, 2019 at 18:47

Thales Minussi · Accepted Answer · 2019-03-04 18:56:23Z

4

The body coming from API Gateway is a stringified JSON, so you need to parse it and then access the attribute you want.

'use strict';

exports.handler = async (event) => {
  let responseBody = {
    message: "hello",
    key1: JSON.parse(event.body).key1
  };
  const response = {
    statusCode: 200,
    headers: { "Access-Control-Allow-Origin": "*" },
    body: JSON.stringify(responseBody)
  };
  return response;

};

This will do it.

edited Mar 4, 2019 at 18:56

answered Mar 4, 2019 at 17:29

Thales Minussi

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4 Comments

hahaha Over a year ago

Thank you for your answer but when I test with postman, cloudwatch still show me key1 is undefined.

hahaha Over a year ago

Thank you for your answer. but second question's point is I send json data with postman like this {"key1":"value1","key2":"value2","key3":"value3} as far as I know lambda event parameter get my json data. I want to use "key1"'s value. your second sample is not that I want.

hahaha Over a year ago

I am very sorry. I think you misunderstood that's my fault. So I changed paragraph. anyway, thank you for your evidence. but I said that's working well in aws editor console. but when I send same data with postman. it's not work well. that's my problem. Can you test for me one more with postman? I appreciate you.

hahaha Over a year ago

I feel grateful with your test and help. unfortunately, Your fourth code didn't work well both of postman, aws console. I got this error in aws console { "errorMessage": "Cannot read property 'key1' of undefined", "errorType": "TypeError", "stackTrace": [ "exports.handler (/var/task/index.js:6:26)" ] } and postman.

Feiga Lubow · Accepted Answer · 2021-04-22 10:19:46Z

1

you need to pass the parameters in postman as JSON.

in the body tab choose the 'raw' option and then from the combo box select 'JSON' type: then just type :

{
  "key1": "value1",
  "key2": "value2",
  "key3": "value3"
}

and in your lambda function access to it like this:

JSON.parse(event.body).key1

answered Apr 22, 2021 at 10:19

Feiga Lubow

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