Cannot run basic example

[andreipopi]

Hi Evgeny,

I am trying to run your library, but upon fixing basic missing errors (with let cb,pre,post;), I still get errors when I run the basic example provided under "Usage". The error I get is:

`json-schema-traverse/index.js:11
  var pre = (typeof cb == 'function') ? cb : cb.pre || function() {};
                                                ^
TypeError: Cannot read properties of undefined (reading 'pre')`

The following is the code I'm using:

const traverse = require('json-schema-traverse');
const schema = {
  properties: {
    foo: {type: 'string'},
    bar: {type: 'integer'}
  }
};

let cb:any;
let pre:any;
let post:any;
traverse(schema, {cb});

What would be a suggestion to solve this issue?

In addition, I am interested in using the library in order to traverse arbitrary json schemas? Is this library able to deal with json-schemas of arbitrary shapes?