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User-defined functions in ARM template

User-defined functions help simplify and reuse complex expressions in your ARM templates. By encapsulating logic in custom functions, you can improve template readability, maintainability, and scalability—especially when the same logic appears in multiple places. These functions are distinct from the built-in template functions that are always available. To define user-defined functions in Bicep, see User-defined functions in Bicep.

Define the function

Your functions require a namespace value to avoid naming conflicts with template functions. The following example shows a function that returns a unique name:

"functions": [
  {
    "namespace": "contoso",
    "members": {
      "uniqueName": {
        "parameters": [
          {
            "name": "namePrefix",
            "type": "string"
          }
        ],
        "output": {
          "type": "string",
          "value": "[concat(toLower(parameters('namePrefix')), uniqueString(resourceGroup().id))]"
        }
      }
    }
  }
],

Use the function

The following example shows a template that includes a user-defined function to get a unique name for a storage account. The template has a parameter named storageNamePrefix that is passed as a parameter to the function.

{
 "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
 "contentVersion": "1.0.0.0",
 "parameters": {
   "storageNamePrefix": {
     "type": "string",
     "maxLength": 11
   }
 },
 "functions": [
  {
    "namespace": "contoso",
    "members": {
      "uniqueName": {
        "parameters": [
          {
            "name": "namePrefix",
            "type": "string"
          }
        ],
        "output": {
          "type": "string",
          "value": "[format('{0}{1}', toLower(parameters('namePrefix')), uniqueString(resourceGroup().id))]"
        }
      }
    }
  }
],
 "resources": [
   {
     "type": "Microsoft.Storage/storageAccounts",
     "apiVersion": "2025-01-01",
     "name": "[contoso.uniqueName(parameters('storageNamePrefix'))]",
     "___location": "South Central US",
     "sku": {
       "name": "Standard_LRS"
     },
     "kind": "StorageV2",
     "properties": {
       "supportsHttpsTrafficOnly": true
     }
   }
 ]
}

During deployment, the storageNamePrefix parameter is passed to the function:

  • The template defines a parameter named storageNamePrefix.
  • The function uses namePrefix because you can only use parameters defined in the function. For more information, see Limitations.
  • In the template's resources section, the name element uses the function and passes the storageNamePrefix value to the function's namePrefix.

Limitations

When defining a user function, there are some restrictions:

  • The function can't access variables.
  • The function can only use parameters that are defined in the function. When you use the parameters function within a user-defined function, you're restricted to the parameters for that function.
  • The function can't call other user-defined functions.
  • The function can't use the reference function or any of the list functions.
  • Parameters for the function can't have default values.

Next steps