Azure DevOps YAML: How to convert an array into formatted string?

In my Azure DevOps YAML pipeline, I have a parameter defined as an array:

parameters:
  - name: variablesToSubstitute
    type: object
    default:
      - test1
      - test2

I need to convert this array into a multiline string where each element follows the format key=$(key). The desired result should look like this:

variables:
  result: |
    test1=$(test1)
    test2=$(test2)

or this:

variables:
  result: test1=$(test1) test2=$(test2)

How can I achieve this in Azure DevOps YML? Is there a way to loop through the array and format each element into this structure?

You can use the join syntax to merge your array with a line break.

variables:
  result: ${{ join('\n', parameters.variablesToSubstitute) }}

steps:
- script: printf "$(result)"

To apply a format to each item, there's a limitation with ${{ each }} in that it cannot appear inside an expression that is a scalar value. Best I can do is pass the formatted values into a template and then join them.

# template: concatenate-variables
parameters:
- name: items
  type: object

- name: variable
  type: string

variables:
- name: ${{ parameters.variableName }}
  value: ${{ join('\n', parameters.items }}

# pipeline
parameters:
- name: variablesToSubstitute
  type: object
  default:
  - test1
  - test2

variables:
- template: concatenate-variables.yml
  parameters:
    items:
    - ${{ each item in parameters.variablesToSubstitute }}:
      - ${{ format('{0}=$({0})', item) }}
    name: result

steps:
- script: printf "$(result)"