Understanding the addProperty() Expression in Power Automate (With Examples)

Introduction

When working with objects in Power Automate, there are times when you need to add a new property dynamically — especially when shaping JSON, preparing data for APIs, or building custom outputs.
This is where the addProperty() expression becomes extremely useful.

In this blog, I’ll explain what it is, how it works, and a real-world example of combining data, adding a new column with addProperty().

What is addProperty() in Power Automate?

addProperty() is a Power Automate expression used to add a new key–value pair to an existing object without modifying the original object.

It returns a new object with the additional property.

Syntax:

addProperty(object, propertyName, propertyValue)

Parameters:

Parameter	Description
object	The base object where you want to add the property
propertyName	Name of the property you want to add
propertyValue	Value you want to store

Example:

Scenario

I have a JSON with FirstName and LastName, and I want to dynamically add a third field, FullName, using addProperty().

Flow Design

Step 1: Manually Trigger the Flow

• Use Manually trigger a flow.

• This allows you to test the logic on demand without dependencies.

Step 2: Initialize an Array Variable

• Add Initialize variable - varRecords

• Name: varRecords

• Type: Array

• Value:

[
  {
    "FirstName": "John",
    "LastName": "Doe"
  },
  {
    "FirstName": "Alice",
    "LastName": "Brown"
  },
  {
    "FirstName": "Michael",
    "LastName": "Smith"
  }
]

Step 3: Initialize an Array Variable

• Add Initialize variable - varFinalOutput

• Name: varFinalOutput

• Type: Array

• Value: (leave empty)

Step 4: Apply to Each

• Add Apply to each

• Input: Select an output from previous step 2

  variables('varRecords')

Step 5 : Add Actions inside Apply to each

• Add Compose - FullName

  • Input:

    [
      {
        "FullName": "John Doe"
      },
      {
        "FullName": "Alice Brown"
      },
      {
        "FullName": "Michael Smith"
      }
    ]

• Add another Compose - Add Column dynamically

  • Input:

    addProperty(item(),
      'FullName',
      coalesce(
      first(outputs('Compose_-_FullName'))?['FullName'],
      ''
      )  
    )

• Add Append to array variable

  • Name: Select varFinalOutput

  • Value: Output of Compose - Add Column dynamically

    outputs('Compose_-_Add_column_dynamically')

Step 9: Final Compose

• Add Compose - Final Output

• Input: Select the varFinalOutput variable

variables('varFinalOutput')

Final Output will be like below JSON:

[
    {
        "FirstName": "John",
        "LastName": "Doe",
        "FullName": "John Doe"
    },
    {
        "FirstName": "Alice",
        "LastName": "Brown",
        "FullName": "John Doe"
    },
    {
        "FirstName": "Michael",
        "LastName": "Smith",
        "FullName": "John Doe"
    }
]

Conclusion

addProperty() allows you to dynamically add new fields to an object in Power Automate without modifying the source data. It is especially useful for merging data from multiple lists, shaping JSON outputs, and creating flexible data structures for downstream actions like Select, Compose, or API calls.