Why Boolean doesn't work as a checked value

gistfile1.js

function getPlayer()
{
    return {
        isWorthy: true
    }
}

function determineIfWorthy(player)
{
    return player.isWorthy();
}

var players = [getPlayer(), getPlayer(), getPlayer()];
console.log(players.map(determineIfWorthy));

// Uncaught TypeError: player.isWorthy is not a function