weightedrandom.js

const slots = [50, 40, 200, 300, 400].sort((a, b) => a < b);
const slotSum = slots.reduce((sum, s) => sum + s, 0);
const random = () => {
    const r = Math.random() * slotSum;
    for (let i = 0, c = 0; i < slots.length; i++) {
        c += slots[i];
        if (r < c) {
            return i;
        }
    }
};

let values = {};
for (let i = 0; i < 10000; i++) {
    const r = Math.floor(random() * 10) / 10;
    values[r] = (values[r] || 0) + 1;
}
Object.keys(values).sort().map(key => {
   return key + ': ' + values[key]; 
});