jwt-decode.js

const atob = window.atob;

function b64DecodeUnicode(str) {
    return decodeURIComponent(atob(str).replace(/(.)/g, function (m, p) {
        let code = p.charCodeAt(0).toString(16).toUpperCase();
        if (code.length < 2) {
            code = '0' + code;
        }
        return '%' + code;
    }));
}

function base64UrlDecode(str) {
    let output = str.replace(/-/g, "+").replace(/_/g, "/");
    switch (output.length % 4) {
        case 0:
            break;
        case 2:
            output += "==";
            break;
        case 3:
            output += "=";
            break;
        default:
            throw "Illegal base64url string!";
    }

    try{
        return b64DecodeUnicode(output);
    } catch (err) {
        return atob(output);
    }
}

export function jwtDecode(token) {
    const payloadPosition = 1;

    return JSON.parse(base64UrlDecode(token.split('.')[payloadPosition]))
}