First, you need to set up an API Client in commercetools. Log into your commercetools account and navigate to the "API Clients" section in the Merchant Center. Create a new API client with the necessary permissions to read the data you need, such as "view_products" for product data. Note down the Client ID, Client Secret, and Project Key as you will need these to authenticate API requests.

Ensure you have `Node.js` and `npm` installed on your local machine. These tools will be used to run scripts that interact with the commercetools API. You can download them from [nodejs.org](https://nodejs.org/).

Open your terminal and create a new directory for your project. Navigate into this directory and initialize a new Node.js project by running:
```bash
npm init -y
```
This will create a `package.json` file which will manage your project's dependencies.

Install the commercetools JavaScript SDK to simplify API interactions. Run the following command in your terminal:
```bash
npm install @commercetools/sdk-client @commercetools/api-request-builder
```
These packages will allow you to authenticate and build requests to the commercetools API.

Create a new JavaScript file, e.g., `fetchData.js`. Use the commercetools SDK to authenticate and fetch the required data. Here's a basic template for fetching product data:
```javascript
const { createClient } = require('@commercetools/sdk-client');
const { createAuthMiddlewareForClientCredentialsFlow } = require('@commercetools/sdk-middleware-auth');
const { createHttpMiddleware } = require('@commercetools/sdk-middleware-http');
const { createApiBuilderFromCtpClient } = require('@commercetools/api-request-builder');
const fetch = require('node-fetch');

const projectKey = 'YOUR_PROJECT_KEY';
const clientId = 'YOUR_CLIENT_ID';
const clientSecret = 'YOUR_CLIENT_SECRET';

const client = createClient({
middlewares: [
createAuthMiddlewareForClientCredentialsFlow({
host: 'https://auth.europe-west1.gcp.commercetools.com',
projectKey,
credentials: {
clientId,
clientSecret,
},
fetch,
}),
createHttpMiddleware({
host: 'https://api.europe-west1.gcp.commercetools.com',
fetch,
}),
],
});

const apiRoot = createApiBuilderFromCtpClient(client).withProjectKey({ projectKey });

apiRoot.products().get().execute()
.then(response => {
const products = response.body.results;
console.log(products);
})
.catch(error => console.error(error));
```

Modify the script to save the fetched data into a local JSON file. You can use Node.js's filesystem module to write data to a file:
```javascript
const fs = require('fs');

apiRoot.products().get().execute()
.then(response => {
const products = response.body.results;
fs.writeFileSync('products.json', JSON.stringify(products, null, 2));
console.log('Data saved to products.json');
})
.catch(error => console.error(error));
```

Finally, run your script to fetch data from commercetools and save it to a local JSON file. In your terminal, execute the following command:
```bash
node fetchData.js
```
If the script runs successfully, you will find a `products.json` file in your project directory containing the fetched data.

By following these steps, you will be able to securely and efficiently transfer data from commercetools into a locally stored JSON file without relying on third-party connectors or integrations.