Disclaimer: I'm the author of jgeXml.
jgexml has Node.js based utility xsd2json which does a transformation between an XML schema (XSD) and a JSON schema file.
As with other options, it's not a 1:1 conversion, and you may need to hand-edit the output to improve the JSON schema validation, but it has been used to represent a complex XML schema inside an OpenAPI (swagger) definition.
A sample of the purchaseorder.xsd given in another answer is rendered as:
"PurchaseOrderType": {
"type": "object",
"properties": {
"shipTo": {
"$ref": "#/definitions/USAddress"
},
"billTo": {
"$ref": "#/definitions/USAddress"
},
"comment": {
"$ref": "#/definitions/comment"
},
"items": {
"$ref": "#/definitions/Items"
},
"orderDate": {
"type": "string",
"pattern": "^[0-9]{4}-[0-9]{2}-[0-9]{2}.*$"
}
},