How promises works

promise.js

function asyncFunction(x) {
    return new Promise(function (resolve, reject) {
        if(typeof x === 'number' && !isNaN(x)){
            resolve( x * x)
        } else {
            reject(new Error('It\'s not a number'))
        }
    })
    
}

asyncFunction(10)
    .then((result)=> console.log(result))
    .catch((error)=> console.error(error))