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If you need to access the query string or path parameters in an API Gateway event you can do so by reading the attributes in event.queryStringParameters, event.multiValueQueryStringParameters and event.pathParameters, for example: event.pathParameters.userId. Unfortunately if there are no parameters for these parameter holders, the relevant key queryStringParameters, multiValueQueryStringParameters or pathParameters won't be available in the object, causing an expression like event.pathParameters.userId to fail with the error: TypeError: Cannot read property 'userId' of undefined.

A simple solution would be to add an if statement to verify if the pathParameters (or queryStringParameters/multiValueQueryStringParameters) exists before accessing one of its parameters, but this approach is very verbose and error prone.

This middleware normalizes the API Gateway event, making sure that an object for queryStringParameters, multiValueQueryStringParameters and pathParameters is always available (resulting in empty objects when no parameter is available), this way you don't have to worry about adding extra if statements before trying to read a property and calling event.pathParameters.userId will result in undefined when no path parameter is available, but not in an error.

Important note : API Gateway HTTP API format 2.0 doesn't have multiValueQueryStringParameters fields. Duplicate query strings are combined with commas and included in the queryStringParameters field.


To install this middleware you can use NPM:

npm install --save @middy/http-event-normalizer

Sample usage

import middy from '@middy/core'
import httpEventNormalizer from '@middy/http-event-normalizer'

const handler = middy((event, context) => {
console.log(`Hello user ${event.pathParameters.userId}`)
// might produce `Hello user undefined`, but not an error

return {}