Create a Connected App with consumer secret using jsforce and consumer client.

create_connectedApp.js

let jsforce = require('jsforce');
let conn = new jsforce.Connection({
    // you can change loginUrl to connect to sandbox or prerelease env.
    // loginUrl : 'https://test.salesforce.com'
});

let username = 'YOUR_USERNAME';
let password = 'YOUR_PASSWORD';
let fullName = 'FULL_NAME'; // no spaces

conn.login(username, password, function (err, userInfo) {
    if (err) {
        return console.error(err);
    }

    let metadata = [{
        contactEmail: 'YOUR_EMAIL',
        description: 'YOUR_DESCRIPTION',
        fullName: fullName, 
        label: 'YOUR_LABEL',
        oauthConfig: {
            callbackUrl: 'YOUR_CALLBACK',
            consumerSecret: 'XXXXXXXX', // at least 8 characters
                                        // unique (and only alphanumeric)
                                        // random number should be ok
            scopes: [
                'Basic',
                'Api',
                'Web',
                'Full',
                'RefreshToken'
            ]
        }
    }];

    conn.metadata.create('ConnectedApp', metadata, function (err, results) {
        if (err) {
            return console.error(err);
        }

        conn.metadata.read('ConnectedApp', fullName, function (err, metadata) {
            if (err) {
                return console.error(err);
            }

            console.log('consumer key: ' + metadata.oauthConfig.consumerKey);
        });
    });
});