interface stsInterface {
    [index: string]: string
}

const sts: stsInterface = {
    UserId: "AAAAAanbc:botocore-session",
    Account: "123456789012",
    Arn: "arn:aws:sts::123456789012:assumed-role/abc",
    Account1: "123456789012",
    Account2: "123456789012",
    Account3: "123456789012",
}
// find the key when value equal '123456789012';
// filter() will return Array back.
// => not give "{}" will return value.
const accountId2 = Object.keys(sts).filter( array => sts[array] === '123456789012');

// find the key when value equal '123456789012';
let accountId: string[] = [];
Object.entries(sts).forEach( array => array[1] == '123456789012' ? accountId.push(array[0]) : false);

console.log(accountId2);
console.log('*'.repeat(50));
console.log(accountId);
//[ 'Account', 'Account1', 'Account2', 'Account3' ]
//**************************************************
//[ 'Account', 'Account1', 'Account2', 'Account3' ]

// console.log(Object.entries(sts))
//[
//  [ 'UserId', 'AAAAAanbc:botocore-session' ],
//  [ 'Account', '123456789012' ],
//  [
//    'Arn',
//    'arn:aws:sts::123456789012:assumed-role/abc'
//  ],
//  [ 'Account1', '123456789012' ],
//  [ 'Account2', '123456789012' ],
//  [ 'Account3', '123456789012' ]
//]