US President Donald Trump boasted of a "phenomenal victory" to cheering supporters at a rally in Iowa on Thursday after Congress narrowly passed his signature tax and spending bill, cementing his radical second-term agenda.

The bill entails tax cuts of about $4.5 trillion and cutbacks in social security programmes such as Medicaid and food stamps.

Congress passed the tax cut and spending megabill with President Trump's legislative priorities which includes new tax exemptions for tipped hourly workers. But how will it work?