Interface in typescript and it's conversion into Javascript

No, interfaces don't exist in Javascript at all. They are only for the compiler.

Although your question does not state this, I am assuming that you would like to convert the compile time check to a runtime one in some automatic way. With that in mind I have found these notes that I will use as a basis for this answer.

Starting with your example and embellishing what Textstyle might be, we create xyz.ts:

interface Textstyle {
  bold: boolean;
  font: string;
}

export interface xyz {
  something?: string;
  somethingStyle?: Textstyle;
}

then install the rest of the dependencies for our example (below is a shell session):

$ echo '{"private":true}' >package.json
$ npm install --save-dev typescript @tsconfig/recommended ts-json-schema-generator
$ echo '{"extends":"@tsconfig/recommended/tsconfig.json"}' >tsconfig.json
$ npx ts-json-schema-generator --path 'xyz.ts' >xyz_schema.json

If we look at the output from ts-json-schema-generator in xyz_schema.json we find:

{
  "$ref": "#/definitions/xyz",
  "$schema": "http://json-schema.org/draft-07/schema#",
  "definitions": {
    "xyz": {
      "additionalProperties": false,
      "properties": {
        "something": {
          "type": "string"
        },
        "somethingStyle": {
          "additionalProperties": false,
          "properties": {
            "bold": {
              "type": "boolean"
            },
            "font": {
              "type": "string"
            }
          },
          "required": [
            "bold",
            "font"
          ],
          "type": "object"
        }
      },
      "type": "object"
    }
  }
}

As JSON schema is a defined format we can use a number of tools to validate our objects against it. Here is something I put together with Ajv:

const Ajv = require("ajv");
const schema = require("./xyz_schema.json");

const ajv = new Ajv();
const validate = ajv.compile(schema);

const x1 = {
  something: "hello world",
  somethingStyle: {
    bold: true,
    font: "comic sans",
  },
};

const x2 = {
  wibbles: "wobble",
};

const testCases = [x1, x2];

testCases.forEach((x, i) => {
  const valid = validate(x);
  if (valid) {
    console.log(`test case ${i} is valid`);
  } else {
    console.error(`test case ${i} is invalid`);
    console.error(validate.errors);
  }
});

Which outputs:

test case 0 is valid
test case 1 is invalid
[
  {
    instancePath: '',
    schemaPath: '#/definitions/xyz/additionalProperties',
    keyword: 'additionalProperties',
    params: { additionalProperty: 'wibbles' },
    message: 'must NOT have additional properties'
  }
]