var data = [
    { date: "2017-04-20", price: 1170 },
    { date: "2017-04-21", price: 1450 },
    { date: "2017-04-22", price: 960 },
    { date: "2017-04-25", price: 2300 },
    { date: "2017-05-21", price: 1203 },
    { date: "2017-05-22", price: 1206 },
    { date: "2017-03-20", price: 658 }
];
var monthMap = {};
data.forEach(function (n) {
    var month = n.date.substring(0, 7);
    if (!monthMap[month] || monthMap[month] > n.price) {
        monthMap[month] = n.price;
    }
});
// {2017-04: 960, 2017-05: 1203, 2017-03: 658}
console.log(monthMap);

var arr = [
    {
        date: '2017-04-20',
        price: 1170
    },
    {
        date: '2017-04-21',
        price: 1450
    },
    {
        date: '2017-04-22',
        price: 940
    }
];

var newArr = arr.sort(function (a, b) {
    return a.price > b.price;
});
console.log(newArr[0].price); // -> 940

First sort the array by price from small to large,
Then get the first content of the array which is the one with the smallest price.