Retrieve the value within a "foo.bar" string in an AWS Lambda payload

Question

In an AWS Step Function, I have a Lambda function whose output looks like this:

{
"foo.bar": "Something"
}

In the next Lambda function, I need to pass this value through the payload.

{
"value.$": "$.foo.bar"
}

When I do this, the step doesn't find the value. I think it might be because it expects something like this:

{
"foo": "bar"
}

I didn't really see a solution in the AWS documentation. When looking for help from AI, I found things like:

{
"value.$": "$.foo.bar.$"
}

... And this didn't work.

Answer 1

When you have a period in your key name like that, you need to use the bracket notation as in the example below. Otherwise, the JSONPath interpreter thinks you are looking for something like:

{
  "foo": {
    "bar": "Something"         
  }
} 

In this example, the first choice state uses the dot notation as you had and doesn't find the key. The second one uses bracket notation and does.

{
  "StartAt": "Simulate Lambda Function",
  "States": {
    "Simulate Lambda Function": {
      "Type": "Pass",
      "Result": {
        "foo.bar": "somevalue"
      },
      "Next": "Try to find using dot notation"
    },
    "Try to find using dot notation": {
      "Type": "Choice",
      "Choices": [
        {
          "Variable": "$.foo.bar",
          "IsPresent": true,
          "Next": "Found It!"
        }
      ],
      "Default": "Try to find using bracket notation"
    },
    "Try to find using bracket notation": {
      "Type": "Choice",
      "Choices": [
        {
          "Variable": "$['foo.bar']",
          "IsPresent": true,
          "Next": "Found It!"
        }
      ],
      "Default": "Didn't find it. :-("
    },
    "Found It!": {
      "Type": "Succeed"
    },
    "Didn't find it. :-(": {
      "Type": "Fail"
    }
  }
}