aws cloudformation use Fn::Join in a list

Question

I've a cloudformation template that uses custom resource backed by lambda function. One of the parameters of the lambda function is a list of strings. I have only one item to pass in the list and would like to use Fn:Join to concatenate create the string. However, using Fn::Join gives error as it leads to invalid json. Any inputs are appreciated.

"Subscriptions": [ "Fn::Join": [":", ["a", "b", "c"]]]

A client error (ValidationError) occurred when calling the CreateStack operation : Template format error: JSON not well-formed.

Cloudformation snippet:-

  "Resources": {
"MyCustomRes": {
      "Type": "Custom::CustomResource",
      "Properties": {
        "ServiceToken": { "Fn::Join": [ "", [
                                        "arn:aws:lambda:",
                                        { "Ref": "AWS::Region" },
                                        ":",
                                        { "Ref": "AWS::AccountId" },
                                        ":function:LambdaFn"
                                      ] ] },
        "Version": 1,
        "ResourceName": { "Ref": "ResourceName" },
        "Subscriptions"       : [ "Fn::Join": [ "", [
                                        "arn:aws:sns:",
                                        { "Ref": "AWS::Region" },
                                        ":",
                                        { "Ref": "AWS::AccountId" },
                                        ":Topic1"
                                      ] ] ]
    }
}     },

Answer 1

The Fn::Join Intrinsic Function used to build the values for the Subscriptions property must be an object rather than an array.

It's invalid JSON syntax to declare an array like ['Fn::Join' : [...]] instead it must be of the form {"Fn::Join" : [...]}

The docs describe the syntax as

{ "Fn::Join" : [ "delimiter", [ comma-delimited list of values ] ] }

Therefore your CloudFormation template should use the following

{
  "Subscriptions": {
    "Fn::Join": [
      ":",
      [
        "arn:aws:sns",
        {
          "Ref": "AWS::Region"
        },
        {
          "Ref": "AWS::AccountId"
        },
        "Topic1"
      ]
    ]
  }
}

A more readable solution to constructing ARN exists using the Fn::Sub Intrinsic Function.

{
  "Fn::Sub": [
    "arn:${AWS::Partition}:sns:${AWS::Region}:${AWS::AccountId}:Topic1"
  ]
}