NEW YORK — On January 3, the S&P 500 sank 2.5 percent when Apple warned of sagging demand for the iPhone, an inauspicious start to 2019 following a 14 percent drubbing in last year’s fourth quarter.

On January 4, Federal Reserve Chairman Jay Powell said the central bank would be “patient” with its interest rate policy following four increases in 2018. The S&P 500 soared 3.4 percent and by the end of the month was up nearly 8 percent.

January’s swing helped set the tone for a year in which the market responded to every downturn with a more sustained upswing. Along the way, stocks kept setting records — 32 of them for the S&P 500 by Dec. 20, and 19 for the Dow Jones Industrial Average.

By its final policy meeting in December, the Fed had completely reversed course and cut rates three times in what Powell called a pre-emptive move against any impact a sluggish global economy and the U.S.-China trade war might have on U.S. economic growth. The stock market, and most Fed observers not named Trump, approved of the Fed’s actions.

Investors’ uncertainty over trade policy eased by December as Washington and Beijing reached a modest, interim agreement that averted a new round of tariffs on $160 billion worth of Chinese imports and reduced existing import taxes on about $112 billion in other Chinese goods.

While the pact left unresolved some of the thorniest issues between the two countries, investors appeared happy to have a de-escalation in trade tensions now and push off lingering concerns until 2020.

Through it all, the U.S. economy and consumers’ appetite for spending remained resilient, supporting the market’s record-shattering, year-end rally.