How to test a function in var in javascript

Given a piece of code like below, which is adopted from reinforcejs

var RL = {};
(function(global) {
    var sampleWeighted = function(p) {
        var r = Math.random();
        var c = 0.0;            // cumulative prob
        for(var i=0, n=p.length; i<n; i++) {
            c += p[i];
            if (c >= r) { return i; }
        }
        // assert(false) may happen if sum(p) < 1;
        assert(false, 'wtf');
    };

// many content omitted
...
})(RL);

export default RL;

How can I access sampleWeighted and test it, please? I have tried below

import RL from './rl.js';

it('sampleWeighted', () => {
    expect(RL.sampleWeighted([0.5, 0.5])).toEqual(1);
});

But it complains with error

FAIL  src/lib/rl.test.js
  ✕ sampleWeighted (1ms)

  ● sampleWeighted

    TypeError: Cannot read property 'sampleWeighted' of undefined

      at Object.<anonymous> (src/lib/rl.test.js:4:51)
      at process._tickCallback (internal/process/next_tick.js:103:7)

I am new to javascript testing. Currently, I am using create-react-app to setup the testing framework. So what's the proper way to test the above function, please? If you happened to understand what sampleWeighted does, please feel free to comment on it, too.