const getTodos = (res) =>{
    return new Promise((resolve, reject) =>{
        const req = new XMLHttpRequest();

        req.addEventListener('readystatechange', ()=>{
            if(req.readyState === 4 && req.status === 200){
                const data = JSON.parse(req.responseText)
                resolve(data)
            }else if(req.readyState === 4){
                reject('could not fetch data')
            }
        })
        req.open('GET', res)
        req.send()
    })
}

getTodos('todos/astra.json').then(data=>{
    console.log(data)
    return getTodos('todos/breach.json')
}).then(data=>{
    console.log(data)
    return getTodos('todos/astra.json')
}).then(data=>{
    console.log(data)
}).catch(err=>{
    console.log(err)
})