Function to generate alpha-numeric strings. First parameter is the length, second parameters is the number of numeric characters.

getUniqAlphaNum.js

const getUniq = (digits, nbTotalNumerics) => {
    let numerics = '0123456789';
    let alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
    let nbNumerics = 0;
    let uniqCode = '';

    for (let i = digits; i > 0; --i) {
        let forceNumeric = (i <= nbTotalNumerics && nbNumerics < nbTotalNumerics);

        if ((forceNumeric || (Math.floor(Math.random() * 2) + 0) === 0) && nbNumerics < nbTotalNumerics) {
            uniqCode += '' + numerics[ Math.floor(Math.random() * numerics.length) ];
            nbNumerics++;
        } else {
            uniqCode += '' + alphabet[ Math.floor(Math.random() * alphabet.length) ];
        }
    }

    return uniqCode;
};