With benefit of hindsight, what’s extraordinary is how few professional investors saw it coming. Mr. Trump was derided as the candidate of “uncertainty,” which markets typically abhor, and many of his stated policies are vague, incoherent or inconsistent. But there was nothing uncertain about his overall pro-growth, pro-business and America-first tendencies, now backed by the firepower of a Republican House and Senate.

He is, after all, a real estate developer.

“We see tremendous opportunity for economic growth,” said John Engler, a former governor of Michigan who is now president of the Business Roundtable, an influential group of chief executives that was often at odds with Mr. Trump during the campaign, especially over trade and immigration. Now that the results are in, though, Mr. Engler sees a silver lining. “The Republicans understand,” he said, “that they’re on the spot to produce results.”

Simon Lack, founder of SL Advisors, an investment advisory firm and operator of a mutual fund that focuses on energy, carried the theme further. “Trump’s win is unambiguously positive” for many sectors of the economy, “especially energy infrastructure,” he said.

The doomsayers also ignored a century of market reactions to presidential elections. “We’ve done extensive research that suggests presidential elections don’t affect markets,” said James Stack, president of InvesTech Research. “The reality is that the market is influenced to the greatest extent by economic factors and monetary policy.”

“In almost all technical and macro aspects, this is still a bull market,” Mr. Stack said, and Mr. Trump’s election does not change that.

Markets generally rally the day after a presidential election, said Jeffrey Hirsch, editor of the Stock Traders Almanac, because elections, whatever their outcome, eliminate a measure of uncertainty. “It doesn’t matter if it’s a Republican or Democrat,” he said. Returns tend to be higher when an incumbent president is replaced or the party in power changes, as happened this week. And previous instances of the election of both a Republican president and Republican Congress have been followed, on average, by a first-year performance of 14 percent.

There are more specific reasons as well that investors applauded Mr. Trump’s victory:

David Fuller's view

Trump’s immediate projects are likely to be sensible infrastructure spending, personal and corporate tax cuts, and deregulation.  These could transform sentiment and show positive results much more quickly than some commentators have suggested.  They should also be of considerable long-term benefit to the US economy.