RTFM

The fucking manual shows only signatures without any explanation about the parameters (like Android ;)

Custom Json

steem.broadcast.customJson(wif, requiredAuths, requiredPostingAuths, id, json, function(err, result) {
  console.log(err, result);
});

https://github.com/steemit/steem-js/tree/master/doc#custom-json

Gladfully, the are many examples on the web and DuckDuckGo shown me the way.

Hello resteem

const steem = require("steem");

const account="your_username";
const wif=steem.auth.toWif(account,"your_password",'posting');

const author="freebot";
const permlink="i-learned-how-to-resteem-using-steem-js";

const json = JSON.stringify(['reblog', {
    account: account,
    author: author,
    permlink: permlink
}]);

steem.broadcast.customJson(wif, [], [account], 'follow', json, (err, result) => {
    console.log(err, result);
});

'follow' ...what is this thing doing here?

I could not have guessed it by reading the doc...