AWS Lambda: How To Access POST Parameters (NodeJS)

Situation 1: HTML Form POST With Action URL As API Gateway URL Connected To AWS Lambda Function

Situation 2: HTML Form POST (With File Upload) With Action URL As API Gateway URL Connected To AWS Lambda Function

Lets assume you have a HTML form with file upload. Something like this..

<form 
    method="POST" 
    action="https://some_api_gateway_url.execute-api.ap-south-1.amazonaws.com/default/postParamsDemo"
    enctype="multipart/form-data">

    <label>Your First Name</label>
    <input type="text" name="first_name" />
    
    <label>Your Last Name</label>
    <input type="text" name="last_name" />
    
    <label>Screenshot</label>
    <input type="file" name="file" />
    
    <label>Message</label>
    <textarea name="message"></textarea>
    
    <input type="submit" />

</form>

Please Notice: You have to set enctype=”multipart/form-data” if you are going to upload files using a HTML Form.

Lets look at the RAW Event Data..

If have left the AWS Lamda function the same. And in order to test this, I have selected and uploaded a tiny PNG image. Below is the RAW response..

{
  "version": "1.0",
  "resource": "/postParamsDemo",
  "path": "/default/postParamsDemo",
  "httpMethod": "POST",
  "headers": {
    "Content-Length": "3093",
    "Content-Type": "multipart/form-data; boundary=----WebKitFormBoundaryLhd9dYahIiwQhBJc",
    "Host": "e5p88is0ub.execute-api.ap-south-1.amazonaws.com",
    "User-Agent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.5060.134 Safari/537.36",
    "X-Amzn-Trace-Id": "Root=1-62f501f2-36f6a02732ef919c189b19ad",
    "X-Forwarded-For": "152.57.214.6",
    "X-Forwarded-Port": "443",
    "X-Forwarded-Proto": "https",
    "accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9",
    "accept-encoding": "gzip, deflate, br",
    "accept-language": "en-GB;q=0.8",
    "cache-control": "max-age=0",
    "origin": "http://127.0.0.1:5500",
    "referer": "http://127.0.0.1:5500/",
    "sec-fetch-dest": "document",
    "sec-fetch-mode": "navigate",
    "sec-fetch-site": "cross-site",
    "sec-fetch-user": "?1",
    "sec-gpc": "1",
    "upgrade-insecure-requests": "1"
  },
  "multiValueHeaders": {
    "Content-Length": [
      "3093"
    ],
    "Content-Type": [
      "multipart/form-data; boundary=----WebKitFormBoundaryLhd9dYahIiwQhBJc"
    ],
    "Host": [
      "e5p88is0ub.execute-api.ap-south-1.amazonaws.com"
    ],
    "User-Agent": [
      "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.5060.134 Safari/537.36"
    ],
    "X-Amzn-Trace-Id": [
      "Root=1-62f501f2-36f6a02732ef919c189b19ad"
    ],
    "X-Forwarded-For": [
      "152.57.214.6"
    ],
    "X-Forwarded-Port": [
      "443"
    ],
    "X-Forwarded-Proto": [
      "https"
    ],
    "accept": [
      "text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9"
    ],
    "accept-encoding": [
      "gzip, deflate, br"
    ],
    "accept-language": [
      "en-GB;q=0.8"
    ],
    "cache-control": [
      "max-age=0"
    ],
    "origin": [
      "http://127.0.0.1:5500"
    ],
    "referer": [
      "http://127.0.0.1:5500/"
    ],
    "sec-fetch-dest": [
      "document"
    ],
    "sec-fetch-mode": [
      "navigate"
    ],
    "sec-fetch-site": [
      "cross-site"
    ],
    "sec-fetch-user": [
      "?1"
    ],
    "sec-gpc": [
      "1"
    ],
    "upgrade-insecure-requests": [
      "1"
    ]
  },
  "queryStringParameters": null,
  "multiValueQueryStringParameters": null,
  "requestContext": {
    "accountId": "383392943",
    "apiId": "e5p88is0ub",
    "domainName": "e5p88is0ub.execute-api.ap-south-1.amazonaws.com",
    "domainPrefix": "e5p88is0ub",
    "extendedRequestId": "Ws093hJlBcwEJRw=",
    "httpMethod": "POST",
    "identity": {
      "accessKey": null,
      "accountId": null,
      "caller": null,
      "cognitoAmr": null,
      "cognitoAuthenticationProvider": null,
      "cognitoAuthenticationType": null,
      "cognitoIdentityId": null,
      "cognitoIdentityPoolId": null,
      "principalOrgId": null,
      "sourceIp": "152.57.214.6",
      "user": null,
      "userAgent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.5060.134 Safari/537.36",
      "userArn": null
    },
    "path": "/default/postParamsDemo",
    "protocol": "HTTP/1.1",
    "requestId": "Ws093hJlBcwEJRw=",
    "requestTime": "11/Aug/2022:13:19:46 +0000",
    "requestTimeEpoch": 1660223986164,
    "resourceId": "ANY /postParamsDemo",
    "resourcePath": "/postParamsDemo",
    "stage": "default"
  },
  "pathParameters": null,
  "stageVariables": null,
  "body": "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",
  "isBase64Encoded": true
}

So it looks like, all the form data was again base64 encoded. The file we uploaded is also in there somewhere.

If you try to decode the base64 data, you get something like..

If you look at how the browser constructs HTTP POST requests, you will see that this is the format used in the case of multipart/form-data

So, how do we parse this?

This is going to be more than just a few lines of code. So, I have created a little node module you can use.

const fs = require('fs');

function getValueIgnoringKeyCase(object, key) {
    const foundKey = Object
        .keys(object)
        .find(currentKey => currentKey.toLocaleLowerCase() === key.toLowerCase());
    return object[foundKey];
}

function getBoundary(event) {
    return getValueIgnoringKeyCase(event.headers, 'Content-Type').split('=')[1];
}

function getFilePath(event, fileName) {
    return `/tmp/${Date.now()}-${fileName}`;
}

function saveFile(buffer, filePath) {
    fs.writeFileSync(filePath, buffer);
}

function multiPartParser(event) {
    const boundary = getBoundary(event);
    const result = {};
    event.body
        .split(boundary)
        .forEach(item => {
            if (/filename=".+"/g.test(item)) {

                let fileContents = item.slice(item.search(/Content-Type:\s.+/g) + item.match(/Content-Type:\s.+/g)[0].length + 4, -4);
                let fileBuffer = Buffer.from(fileContents, 'binary');
                
                let fileName = item.match(/filename=".+"/g)[0].slice(10, -1);
                let filePath = getFilePath(event, fileName);
                saveFile(fileBuffer, filePath);

                result[item.match(/name=".+";/g)[0].slice(6, -2)] = {
                    filename: fileName,
                    contentType: item.match(/Content-Type:\s.+/g)[0].slice(14),
                    path: filePath
                }

            } else if (/name=".+"/g.test(item)){
                result[item.match(/name=".+"/g)[0].slice(6, -1)] = item.slice(item.search(/name=".+"/g) + item.match(/name=".+"/g)[0].length + 4, -4);
            }
        });
    return result;
};

module.exports.parse = (event) => {

    let clonedEvent = Object.assign({}, event);
 
    if (event.isBase64Encoded) {
        let body = clonedEvent.body;
        let decodedFromBase64 = Buffer.from(body, 'base64');
        clonedEvent.body = decodedFromBase64.toString('latin1');   
    }
    let result = multiPartParser(clonedEvent);
    
    return result;
}

The above module takes the raw event data as input. It decodes and parses it. And gives back a nice JS Object with the params.

It will also take the file data and store the file in the “tmp” folder. It will give you back a path to the file.

How To Use This Module

Create a new file in the AWS Lambda Code section called “multipart_form_data_parser.js” and paste in the code from above.

You can then call the module from the main AWS Lambda JS function like so..

const multiPartParser = require('./multipart_form_data_parser');

exports.handler = async (event) => {
    
    let postParams = multiPartParser.parse(event);
    
    // TODO implement
    const response = {
        statusCode: 200,
        headers: {"content-type": "application/json"},
        body: JSON.stringify(postParams),
    };
    return response;
};

The response from the AWS function would be something like..

{
  "first_name": "Livefire",
  "last_name": "dev",
  "file": {
    "filename": "file_name.png",
    "contentType": "image/png",
    "path": "/tmp/1660312156425-file_name.png"
  },
  "message": "This is a test message!"
}

As you can see above, the response has the file saved in the “tmp” folder in the Lambda environment. You get back the path of the file so that you can process it in whatever way you like.

As you can see, the “tmp” directory can store files of quite a large size as per this official AWS doc.

$.ajax({
    url: "https://lfdev2123.execute-api.ap-south-1.amazonaws.com/default/postParamsDemo",
    type: "POST",
    crossDomain: true,
    data: JSON.stringify({
        name: 'Livefire',
        domain: 'livefiredev.com'
    }),
    dataType: "json",
    success: function (response) {
        $("#result_json").val(JSON.stringify(response));
    },
    error: function (xhr, status) {
        alert("error");
    }
});

{
  "version": "1.0",
  "resource": "/postParamsDemo",
  "path": "/default/postParamsDemo",
  "httpMethod": "POST",
  "headers": {
    "Content-Length": "46",
    "Content-Type": "application/x-www-form-urlencoded; charset=UTF-8",
    "Host": "e5p88is0ub.execute-api.ap-south-1.amazonaws.com",
    "User-Agent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.5060.134 Safari/537.36",
    "X-Amzn-Trace-Id": "Root=1-62f5ad89-38e3d42b165c3be622e14b14",
    "X-Forwarded-For": "152.57.195.102",
    "X-Forwarded-Port": "443",
    "X-Forwarded-Proto": "https",
    "accept": "application/json, text/javascript, */*; q=0.01",
    "accept-encoding": "gzip, deflate, br",
    "accept-language": "en-GB,en-US;q=0.9,en;q=0.8",
    "origin": "http://127.0.0.1:5500",
    "referer": "http://127.0.0.1:5500/",
    "sec-fetch-dest": "empty",
    "sec-fetch-mode": "cors",
    "sec-fetch-site": "cross-site",
    "sec-gpc": "1"
  },
  "multiValueHeaders": {
    "Content-Length": [
      "46"
    ],
    "Content-Type": [
      "application/x-www-form-urlencoded; charset=UTF-8"
    ],
    "Host": [
      "e5p88is0ub.execute-api.ap-south-1.amazonaws.com"
    ],
    "User-Agent": [
      "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.5060.134 Safari/537.36"
    ],
    "X-Amzn-Trace-Id": [
      "Root=1-62f5ad89-38e3d42b165c3be622e14b14"
    ],
    "X-Forwarded-For": [
      "152.57.195.102"
    ],
    "X-Forwarded-Port": [
      "443"
    ],
    "X-Forwarded-Proto": [
      "https"
    ],
    "accept": [
      "application/json, text/javascript, */*; q=0.01"
    ],
    "accept-encoding": [
      "gzip, deflate, br"
    ],
    "accept-language": [
      "en-GB,en-US;q=0.9,en;q=0.8"
    ],
    "origin": [
      "http://127.0.0.1:5500"
    ],
    "referer": [
      "http://127.0.0.1:5500/"
    ],
    "sec-fetch-dest": [
      "empty"
    ],
    "sec-fetch-mode": [
      "cors"
    ],
    "sec-fetch-site": [
      "cross-site"
    ],
    "sec-gpc": [
      "1"
    ]
  },
  "queryStringParameters": null,
  "multiValueQueryStringParameters": null,
  "requestContext": {
    "accountId": "383322943",
    "apiId": "e5p88is0ub",
    "domainName": "e5p88is0ub.execute-api.ap-south-1.amazonaws.com",
    "domainPrefix": "e5p88is0ub",
    "extendedRequestId": "WugNfiyghcwEJTA=",
    "httpMethod": "POST",
    "identity": {
      "accessKey": null,
      "accountId": null,
      "caller": null,
      "cognitoAmr": null,
      "cognitoAuthenticationProvider": null,
      "cognitoAuthenticationType": null,
      "cognitoIdentityId": null,
      "cognitoIdentityPoolId": null,
      "principalOrgId": null,
      "sourceIp": "152.57.195.102",
      "user": null,
      "userAgent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.5060.134 Safari/537.36",
      "userArn": null
    },
    "path": "/default/postParamsDemo",
    "protocol": "HTTP/1.1",
    "requestId": "WugNfiyghcwEJTA=",
    "requestTime": "12/Aug/2022:01:31:53 +0000",
    "requestTimeEpoch": 1660267913300,
    "resourceId": "ANY /postParamsDemo",
    "resourcePath": "/postParamsDemo",
    "stage": "default"
  },
  "pathParameters": null,
  "stageVariables": null,
  "body": "eyJuYW1lIjoiTGl2ZWZpcmUiLCJkb21haW4iOiJsaXZlZmlyZWRldi5jb20ifQ==",
  "isBase64Encoded": true
}

let encoded = "eyJuYW1lIjoiTGl2ZWZpcmUiLCJkb21haW4iOiJsaXZlZmlyZWRldi5jb20ifQ==";

let getJSONFromRequest = (encodedData) => {

    let decoded = atob(encodedData);
    return JSON.parse(decoded);

}

console.log(getJSONFromRequest(encoded));

{ name: 'Livefire', domain: 'livefiredev.com' }