Variables vs Compose in Power Automate: What’s the Difference and When to Use?

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When building Power Automate flows, two commonly used actions for storing data are:

  • Initialize/Set Variable
  • Compose

🧠 What is a Variable?

A variable is a mutable container — which means you can change or update its value after it’s created.

  • You must initialize a variable with a specific datatype like String, Boolean, Integer, Array, or Object.
  • Later in the flow, you can use Set Variable or Append to Variable actions to update its value.

✅ Use Variables When:

  • You need to update the value multiple times
  • You want to append values (like building a text string or list)
  • You’re using loops or conditions where values change dynamically

📌 Example:
Set a variable “Message” = “Hello”
Later, update “Message” = “Hello, World!”

🔹 What is Compose?

The Compose action is immutable — once set, its value cannot be changed.

  • You do not declare a datatype in Compose. It automatically infers the datatype from the input you provide.
  • It’s great for:
    • Storing a calculated value or expression output
    • Creating short, readable expressions
    • Reducing clutter by avoiding repeated expressions

✅ Use Compose When:

  • You only need the value once
  • The value is static or calculated just once
  • You’re storing the result of a formula or single expression

📌 Example:
Compose = concat(‘Hello ‘, triggerBody()?[‘name’])

Key Differences: Variables vs Compose

FeatureVariableCompose
Can update value later✅ Yes❌ No
Needs initialization✅ Yes (with datatype)❌ No
Declare datatype✅ Required❌ Not required
Performance impact🟠 Slightly heavier (in loops)🟢 Lightweight
Use in loops✅ Ideal for counters/lists🚫 Not recommended
Simplicity🟠 Verbose (init + set)🟢 Simple, one step

💡 Pro Tips:

  • For one-time expressions or values, Compose is cleaner and faster.
  • For accumulating data, counters, or changing values in loops, Variables are necessary.
  • Compose helps avoid repeating expressions and makes your flow easier to read.
  • Always remember: variables require you to specify datatype when initializing, but Compose automatically infers datatype, making it more flexible but immutable.

🔚 Summary:

  • If you’re just holding a value for reference, use Compose.
  • If you’re manipulating or updating data dynamically, use Variables.

Choosing the right one isn’t just good practice — it can make your flow faster, clearer, and easier to maintain.