Quick 'n' dirty id generator

idgen.js

const generateId = (len=16) => {
    const allowedRanges = [[48, 57], [65,90], [97, 122]];
    let str = "";
    let expand = allowedRanges.reduce((acc, each) => {
        let [start, end] = each;
        let nums = [];
        for(let i = start; i <= end; i++) {
            nums.push(i);
        };
        return acc.concat(nums);
    }, []);
    for (let i = 0; i < len; i++) {
        str += String.fromCharCode(
            expand[Math.floor(Math.random() * expand.length)]
        );
    }
    return str;
};