class aws_cdk.CfnJson(scope, id, *, value)

Bases: Construct

Captures a synthesis-time JSON object a CloudFormation reference which resolves during deployment to the resolved values of the JSON object.

The main use case for this is to overcome a limitation in CloudFormation that does not allow using intrinsic functions as dictionary keys (because dictionary keys in JSON must be strings). Specifically this is common in IAM conditions such as StringEquals: { lhs: "rhs" } where you want “lhs” to be a reference.

This object is resolvable, so it can be used as a value.

This construct is backed by a custom resource.




# Example automatically generated from non-compiling source. May contain errors.
tag_param = CfnParameter(self, "TagName")

string_equals = CfnJson(self, "ConditionJson",
        "f"aws:PrincipalTag/{tagParam.valueAsString}"": True

principal = iam.AccountRootPrincipal().with_conditions({
    "StringEquals": string_equals

iam.Role(self, "MyRole", assumed_by=principal)
  • scope (Construct) –

  • id (str) –

  • value (Any) – The value to resolve. Can be any JavaScript object, including tokens and references in keys or values.



Produce the Token’s value at resolution time.



Return type:



This is required in case someone JSON.stringifys an object which refrences this object. Otherwise, we’ll get a cyclic JSON reference.

Return type:



Returns a string representation of this construct.

Return type:




The creation stack of this resolvable which will be appended to errors thrown during resolution.

This may return an array with a single informational element indicating how to get this property populated, if it was skipped for performance reasons.


The tree node.


An Fn::GetAtt to the JSON object passed through value and resolved during synthesis.

Normally there is no need to use this property since CfnJson is an IResolvable, so it can be simply used as a value.

Static Methods

classmethod is_construct(x)

Checks if x is a construct.

Use this method instead of instanceof to properly detect Construct instances, even when the construct library is symlinked.

Explanation: in JavaScript, multiple copies of the constructs library on disk are seen as independent, completely different libraries. As a consequence, the class Construct in each copy of the constructs library is seen as a different class, and an instance of one class will not test as instanceof the other class. npm install will not create installations like this, but users may manually symlink construct libraries together or use a monorepo tool: in those cases, multiple copies of the constructs library can be accidentally installed, and instanceof will behave unpredictably. It is safest to avoid using instanceof, and using this type-testing method instead.


x (Any) – Any object.

Return type:



true if x is an object created from a class which extends Construct.