Use toLowerCase for checking for inclusion of strings

[mattfbacon]

function isWelcomeMessage(message : string) {
    return (message.includes('welcome') || message.includes('Welcome'));
}

can be replaced with

function isWelcomeMessage(message : string) {
    return message.toLowerCase().includes('welcome');
}

(And of course this can be an arrow function but that's up to you.)

[mattfbacon]

This applies to Behavioral/chainOfResponsibility.ts

[OlexG]

but what if I don't want wElCoMe to be considered a welcome message >:)

[mattfbacon]

/^[wW]elcome/.test(message)