# Fn

CloudFormation intrinsic functions.

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference.html

- Source: [`core/lib/cfn-fn.ts`](https://github.com/aws/aws-cdk/blob/v2.275.0/packages/aws-cdk-lib/core/lib/cfn-fn.ts#L15)
- Type last changed: v2.253.0 · 6 May 2026

## Example

```ts
import * as cdk from 'aws-cdk-lib';

declare const destinationBucket: s3.Bucket;

const myBucketDeployment = new s3deploy.BucketDeployment(this, 'DeployMeWithoutExtractingFilesOnDestination', {
  sources: [s3deploy.Source.asset(path.join(__dirname, 'my-website'))],
  destinationBucket,
  extract: false,
});

new cdk.CfnOutput(this, 'ObjectKey', {
  value: cdk.Fn.select(0, myBucketDeployment.objectKeys),
});
```

## Methods

- `base64(data: string): string` (static) — The intrinsic function ``Fn::Base64`` returns the Base64 representation of the input string.

  This function is typically used to pass encoded data to
  Amazon EC2 instances by way of the UserData property.

  **Parameters**

  - `data` — The string value you want to convert to Base64.

  **Returns:** a token represented as a string

- `cidr(ipBlock: string, count: number, sizeMask?: string): string[]` (static) — The intrinsic function ``Fn::Cidr`` returns the specified Cidr address block.

  **Parameters**

  - `ipBlock` — The user-specified default Cidr address block.
  - `count` — The number of subnets' Cidr block wanted.
  - `sizeMask` — The digit covered in the subnet.

  **Returns:** a token represented as a string

- `conditionAnd(...conditions: ICfnConditionExpression[]): ICfnRuleConditionExpression` (static) — Returns true if all the specified conditions evaluate to true, or returns false if any one of the conditions evaluates to false.

  ``Fn::And`` acts as
  an AND operator. The minimum number of conditions that you can include is
  1.

  **Parameters**

  - `conditions` — conditions to AND.

  **Returns:** an FnCondition token

- `conditionContains(listOfStrings: string[], value: string): ICfnRuleConditionExpression` (static) — Returns true if a specified string matches at least one value in a list of strings.

  **Parameters**

  - `listOfStrings` — A list of strings, such as "A", "B", "C".
  - `value` — A string, such as "A", that you want to compare against a list of strings.

  **Returns:** an FnCondition token

- `conditionEachMemberEquals(listOfStrings: string[], value: string): ICfnRuleConditionExpression` (static) — Returns true if a specified string matches all values in a list.

  **Parameters**

  - `listOfStrings` — A list of strings, such as "A", "B", "C".
  - `value` — A string, such as "A", that you want to compare against a list of strings.

  **Returns:** an FnCondition token

- `conditionEachMemberIn(stringsToCheck: string[], stringsToMatch: string[]): ICfnRuleConditionExpression` (static) — Returns true if each member in a list of strings matches at least one value in a second list of strings.

  **Parameters**

  - `stringsToCheck` — A list of strings, such as "A", "B", "C".
  - `stringsToMatch` — A list of strings, such as "A", "B", "C".

  **Returns:** an FnCondition token

- `conditionEquals(lhs: any, rhs: any): ICfnRuleConditionExpression` (static) — Compares if two values are equal.

  Returns true if the two values are equal
  or false if they aren't.

  **Parameters**

  - `lhs` — A value of any type that you want to compare.
  - `rhs` — A value of any type that you want to compare.

  **Returns:** an FnCondition token

- `conditionIf(conditionId: string, valueIfTrue: any, valueIfFalse: any): ICfnRuleConditionExpression` (static) — Returns one value if the specified condition evaluates to true and another value if the specified condition evaluates to false.

  Currently, AWS
  CloudFormation supports the ``Fn::If`` intrinsic function in the metadata
  attribute, update policy attribute, and property values in the Resources
  section and Outputs sections of a template. You can use the AWS::NoValue
  pseudo parameter as a return value to remove the corresponding property.

  **Parameters**

  - `conditionId` — A reference to a condition in the Conditions section.
  - `valueIfTrue` — A value to be returned if the specified condition evaluates to true.
  - `valueIfFalse` — A value to be returned if the specified condition evaluates to false.

  **Returns:** an FnCondition token

- `conditionNot(condition: ICfnConditionExpression): ICfnRuleConditionExpression` (static) — Returns true for a condition that evaluates to false or returns false for a condition that evaluates to true.

  ``Fn::Not`` acts as a NOT operator.

  **Parameters**

  - `condition` — A condition such as ``Fn::Equals`` that evaluates to true or false.

  **Returns:** an FnCondition token

- `conditionOr(...conditions: ICfnConditionExpression[]): ICfnRuleConditionExpression` (static) — Returns true if any one of the specified conditions evaluate to true, or returns false if all of the conditions evaluates to false.

  ``Fn::Or`` acts
  as an OR operator. The minimum number of conditions that you can include is
  1.

  **Parameters**

  - `conditions` — conditions that evaluates to true or false.

  **Returns:** an FnCondition token

- `findInMap(mapName: string, topLevelKey: string, secondLevelKey: string, defaultValue?: string): string` (static) — The intrinsic function ``Fn::FindInMap`` returns the value corresponding to keys in a two-level map that is declared in the Mappings section.

  Warning: do not use with lazy mappings as this function will not guarantee a lazy mapping to render in the template.
  Prefer to use `CfnMapping.findInMap` in general.

  **Returns:** a token represented as a string

- `getAtt(logicalNameOfResource: string, attributeName: string): IResolvable` (static) — The ``Fn::GetAtt`` intrinsic function returns the value of an attribute from a resource in the template.

  **Parameters**

  - `logicalNameOfResource` — The logical name (also called logical ID) of the resource that contains the attribute that you want.
  - `attributeName` — The name of the resource-specific attribute whose value you want.

  **Returns:** an IResolvable object

- `getAzs(region?: string): string[]` (static) — The intrinsic function ``Fn::GetAZs`` returns an array that lists Availability Zones for a specified region.

  Because customers have access to
  different Availability Zones, the intrinsic function ``Fn::GetAZs`` enables
  template authors to write templates that adapt to the calling user's
  access. That way you don't have to hard-code a full list of Availability
  Zones for a specified region.

  **Parameters**

  - `region` — The name of the region for which you want to get the Availability Zones.

  **Returns:** a token represented as a string array

- `getStackOutput(stackName: string, outputName: string, region?: string, roleArn?: string): string` (static) — The intrinsic function `Fn::GetStackOutput` returns the value of an output from another stack.

  This is similar to `Fn::ImportValue`, but it can reference
  a normal output, without the need to export anything. As with `Fn::ImportValue`,
  you typically use this function to create cross-stack references.

  **Parameters**

  - `stackName` — The name of the producer stack.
  - `outputName` — The name of the output to get the value from.
  - `region` — The region where the producer stack is deployed.
  - `roleArn` — The role to be used when describing the producer stack.

- `importListValue(sharedValueToImport: string, assumedLength: number, delimiter?: string): string[]` (static) — Like `Fn.importValue`, but import a list with a known length.

  If you explicitly want a list with an unknown length, call `Fn.split(',',
  Fn.importValue(exportName))`. See the documentation of `Fn.split` to read
  more about the limitations of using lists of unknown length.

  `Fn.importListValue(exportName, assumedLength)` is the same as
  `Fn.split(',', Fn.importValue(exportName), assumedLength)`,
  but easier to read and impossible to forget to pass `assumedLength`.

- `importValue(sharedValueToImport: string): string` (static) — The intrinsic function ``Fn::ImportValue`` returns the value of an output exported by another stack.

  You typically use this function to create
  cross-stack references. In the following example template snippets, Stack A
  exports VPC security group values and Stack B imports them.

  **Parameters**

  - `sharedValueToImport` — The stack output value that you want to import.

  **Returns:** a token represented as a string

- `join(delimiter: string, listOfValues: string[]): string` (static) — The intrinsic function ``Fn::Join`` appends a set of values into a single value, separated by the specified delimiter.

  If a delimiter is the empty
  string, the set of values are concatenated with no delimiter.

  **Parameters**

  - `delimiter` — The value you want to occur between fragments.
  - `listOfValues` — The list of values you want combined.

  **Returns:** a token represented as a string

- `len(array: any): number` (static) — The intrinsic function `Fn::Length` returns the number of elements within an array or an intrinsic function that returns an array.

  **Parameters**

  - `array` — The array you want to return the number of elements from.

- `parseDomainName(url: string): string` (static) — Given an url, parse the domain name.

  **Parameters**

  - `url` — the url to parse.

- `ref(logicalName: string): string` (static) — The ``Ref`` intrinsic function returns the value of the specified parameter or resource.

  Note that it doesn't validate the logicalName, it mainly serves parameter/resource reference defined in a ``CfnInclude`` template.

  **Parameters**

  - `logicalName` — The logical name of a parameter/resource for which you want to retrieve its value.

- `refAll(parameterType: string): string[]` (static) — Returns all values for a specified parameter type.

  **Parameters**

  - `parameterType` — An AWS-specific parameter type, such as AWS::EC2::SecurityGroup::Id or AWS::EC2::VPC::Id.

  **Returns:** a token represented as a string array

- `select(index: number, array: string[]): string` (static) — The intrinsic function ``Fn::Select`` returns a single object from a list of objects by index.

  **Parameters**

  - `index` — The index of the object to retrieve.
  - `array` — The list of objects to select from.

  **Returns:** a token represented as a string

- `split(delimiter: string, source: string, assumedLength?: number): string[]` (static) — Split a string token into a token list of string values.

  Specify the location of splits with a delimiter such as ',' (a comma).
  Renders to the `Fn::Split` intrinsic function.

  Lists with unknown lengths (default)
  -------------------------------------

  Since this function is used to work with deploy-time values, if `assumedLength`
  is not given the CDK cannot know the length of the resulting list at synthesis time.
  This brings the following restrictions:

  - You must use `Fn.select(i, list)` to pick elements out of the list (you must not use
    `list[i]`).
  - You cannot add elements to the list, remove elements from the list,
    combine two such lists together, or take a slice of the list.
  - You cannot pass the list to constructs that do any of the above.

  The only valid operation with such a tokenized list is to pass it unmodified to a
  CloudFormation Resource construct.

  Lists with assumed lengths
  --------------------------

  Pass `assumedLength` if you know the length of the list that will be
  produced by splitting. The actual list length at deploy time may be
  *longer* than the number you pass, but not *shorter*.

  The returned list will look like:

  ```
  [Fn.select(0, split), Fn.select(1, split), Fn.select(2, split), ...]
  ```

  The restrictions from the section "Lists with unknown lengths" will now be lifted,
  at the expense of having to know and fix the length of the list.

  **Parameters**

  - `delimiter` — A string value that determines where the source string is divided.
  - `source` — The string value that you want to split.
  - `assumedLength` — The length of the list that will be produced by splitting.

  **Returns:** a token represented as a string array

- `sub(body: string, variables?: { [key: string]: string }): string` (static) — The intrinsic function ``Fn::Sub`` substitutes variables in an input string with values that you specify.

  In your templates, you can use this function
  to construct commands or outputs that include values that aren't available
  until you create or update a stack.

  **Parameters**

  - `body` — A string with variables that AWS CloudFormation substitutes with their associated values at runtime.
  - `variables` — The name of a variable that you included in the String parameter.

  **Returns:** a token represented as a string

- `toJsonString(object: any): string` (static) — The `Fn::ToJsonString` intrinsic function converts an object or array to its corresponding JSON string.

  **Parameters**

  - `object` — The object or array to stringify.

- `transform(macroName: string, parameters: { [key: string]: any }): IResolvable` (static) — Creates a token representing the ``Fn::Transform`` expression.

  **Parameters**

  - `macroName` — The name of the macro to perform the processing.
  - `parameters` — The parameters to be passed to the macro.

  **Returns:** a token representing the transform expression

- `valueOf(parameterOrLogicalId: string, attribute: string): string` (static) — Returns an attribute value or list of values for a specific parameter and attribute.

  **Parameters**

  - `parameterOrLogicalId` — The name of a parameter for which you want to retrieve attribute values.
  - `attribute` — The name of an attribute from which you want to retrieve a value.

  **Returns:** a token represented as a string

- `valueOfAll(parameterType: string, attribute: string): string[]` (static) — Returns a list of all attribute values for a given parameter type and attribute.

  **Parameters**

  - `parameterType` — An AWS-specific parameter type, such as AWS::EC2::SecurityGroup::Id or AWS::EC2::VPC::Id.
  - `attribute` — The name of an attribute from which you want to retrieve a value.

  **Returns:** a token represented as a string array