Growth isn't everything in investing. Indeed, it only represents a fraction of what comprises a healthy company. But it's undoubtedly an important characteristic to look for when searching for companies worth holding for the long haul. After all, strong growth shows that more and more customers are willing to pay up for a company's products or services.

Two good examples of fast-growing companies are electric-car company Tesla (NASDAQ:TSLA) and programmatic digital advertising platform The Trade Desk (NASDAQ:TTD). Revenue is skyrocketing at both of these companies.

Here's a closer look.

Tesla's Model S, 3, X, and Y

Image source: Tesla.

Tesla

Electric-car maker Tesla saw its revenue surge in 2019, soaring 82% year over year. This revenue growth was primarily driven by a big increase in auto sales as the company started ramping up production of its most affordable vehicle yet: the Model 3. Tesla's total vehicle deliveries jumped 138% year over year in 2018.

Unfortunately, Tesla is still losing money on an annual basis. The capital-intensive nature of the auto business requires massive investment to grow production and revenue and it has been weighing on Tesla's rapid expansion. The company lost $5.72 per share in 2018. But investors should take heart in the fact that Tesla's 2018 loss was significantly narrower than its $11.83 loss per share in 2017. The company's narrowing annual loss is a good sign for Tesla's ability to become profitable as sales continue to rise.

In 2019, Tesla expects more strong growth. The company guided for total vehicle deliveries between 360,000 and 400,000, up 45% to 65% year over year. In addition, Tesla expects to be profitable and free cash flow positive in every quarter beyond the first quarter of 2019.

The Trade Desk

As advertisers have ramped up spending on The Trade Desk's programmatic ad-buying platform, the company's revenue rose 55% year over year last year -- an acceleration from 52% growth in 2017. The Trade Desk benefited from soaring ad spend across its digital channels. Notably, connected TV ad spend increased nine-fold in 2018 compared to 2017. Audio and mobile video ad spend jumped 230% and 130%, respectively, over the same timeframe.

A person looking at charts on a laptop

Image source: Getty Images.

Growth remained strong as the company exited the year, with revenue rising 56% year over year.

Unlike Tesla, The Trade Desk runs an extremely profitable operation. The company generated $88 million of net income on its $477 million of revenue in 2018, giving The Trade Desk an 18.5% net margin. Even more, this net income is rising sharply, climbing 73% year over year in 2018.

With a fresh overhaul to its platform complete and The Trade Desk's recent move to give marketers access to premium digital advertising inventory in China, strong growth should persist in 2019. Management guided for revenue of "at least" $637 million in 2019, representing 41% year-over-year growth or greater. But The Trade Desk's guidance has historically proved to be conservative.