optional-property.md

Understanding the optional Property in a Babel AST OptionalMemberExpression Node

In a Babel Abstract Syntax Tree (AST), the OptionalMemberExpression node represents an optional chaining operation involving member access. This node is used to describe expressions where you use optional chaining to safely access deeply nested properties of an object without having to explicitly check if each reference in the chain is non-null or undefined.

The optional property in an OptionalMemberExpression node is a boolean that indicates whether the member access is optional.

• If optional is true, it means the member access is part of an optional chain, where the operation will short-circuit if the preceding part of the chain is nullish (null or undefined).
• If optional is false, it means the member access is not optional, and any attempt to access a member on a nullish value will result in an error.

Example

Consider the following code snippet:

const obj = {
  foo: {
    bar: 42,
  },
};

const value = obj?.foo?.bar;

In this example, obj?.foo?.bar uses optional chaining to safely access bar without causing an error if obj or obj.foo is nullish.

The Babel AST representation of this optional chaining operation will include OptionalMemberExpression nodes. Here's a simplified view of how Babel might represent it:

{
  "type": "OptionalMemberExpression",
  "object": {
    "type": "OptionalMemberExpression",
    "object": {
      "type": "Identifier",
      "name": "obj"
    },
    "property": {
      "type": "Identifier",
      "name": "foo"
    },
    "optional": true
  },
  "property": {
    "type": "Identifier",
    "name": "bar"
  },
  "optional": true
}

Explanation of the optional Property

• Outer OptionalMemberExpression: Represents the access to bar (obj?.foo?.bar). The optional property is true, indicating that if obj?.foo is nullish, the access will return undefined instead of causing an error.

• Inner OptionalMemberExpression: Represents the access to foo (obj?.foo). The optional property is also true, indicating that if obj is nullish, the access will return undefined instead of causing an error.

For instance, for the expression obj?.foo.bar the outer optional property would be false and the inner optional property would be true. See /doc/nicolo-howto-talk/optional-chain.md for the corresponding AST.

If the optional property were false in any of these nodes, it would mean that the member access at that level is not using optional chaining, and an attempt to access a property on a nullish value would throw an error.

Summary

In an OptionalMemberExpression node in a Babel AST, the optional property plays a crucial role in determining whether the member access operation is part of an optional chain. This property allows Babel to correctly parse and transform JavaScript code that uses optional chaining, ensuring that the semantics of the original code are preserved during the transformation process.