Market crashes can be stressful for investors. It isn't easy to watch the gains you have accumulated over several months (or years) dwindle before your eyes. But downturns can also serve as opportunities to buy shares of great companies at a discount. Few investors expressly wish for a market crash, but it's important to make the most of it if (or rather, when) it occurs. 

And while nobody can predict precisely the timing of a pullback, having a list of companies that are worth picking up from the discount bin at the ready isn't a bad idea. In that spirit, here are two excellent growth stocks that investors should consider buying during the next downturn: Intuitive Surgical (ISRG 1.62%) and Shopify (SHOP -1.16%). These highly successful businesses would be even better buys if their stocks dipped, considering the expensive levels at which they trade today.

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1. Intuitive Surgical 

Intuitive Surgical has its detractors, and at the moment, it isn't too difficult to find reasons to be skeptical of the company's prospects. For instance, COVID-19 has proven a major problem for the medical devices specialist. According to CFO Marshall Mohr, more cases could lead to even more delayed and postponed surgeries, which would continue to impact the company's top line.

Meanwhile, Intuitive Surgical is trading at 57 times forward earnings. By comparison, the average forward price-to-earnings (P/E) ratio for the S&P 500 is 25.35. Near-term headwinds coupled with high valuation metrics isn't a good combination -- for any company. If these obstacles materialize and affect the company's results in the next few quarters, investors could feel that the stock is overvalued, leading to a sell-off.

But it would be wise to look beyond these issues for one simple reason: Intuitive Surgical's long-term prospects remain more or less intact. The need for minimally invasive surgeries -- the kind the company's da Vinci System makes possible -- will only increase, especially given our aging population.

Robotic-assisted surgery device.

Image source: Getty Images.

With nearly 6,000 da Vinci Systems installed worldwide as of the end of 2020, Intuitive Surgical is well-positioned to profit from the growth of the market for minimally invasive surgeries. The company also doesn't have to worry so much about competitors, since the stringent regulations in the medical device industry serve as a powerful barrier to entry. Further, Intuitive Surgical's revenue is increasingly coming from selling instruments and accessories that go along with its da Vinci System.

Why does that matter? While the company's crown jewel costs between $500,000 and $2.5 million, the sale of its actual robotic-assisted surgery device does not garner attractive margins. Selling the tools to accompany the da Vinci System is what helps the company make money. As Intuitive Surgical's installed bases increase -- and the number of procedures performed with the da Vinci System grows over time -- the company will sell more instruments and accessories, which will work wonders for its top and bottom lines. 

Thanks to these factors, Intuitive Surgical's financial results will only improve, as will its stock price. That's why I think picking up this healthcare stock, particularly if it dips because of a downturn, would be a great move. 

2. Shopify

Shopify's shareholders know a thing or two about sky-high valuation metrics. The tech company is currently trading for a whopping forward P/E ratio of 290. If Shopify were to fall significantly along with the broader market -- thus rendering its stock a bit cheaper -- it would absolutely be worth scooping up its shares. Here's why: Shopify's future is tied to the growth of the e-commerce sector.

As more retail transactions switch from traditional brick-and-mortar stores to online stores, new businesses will increasingly look to build an online presence. Shopify offers merchants a suite of services -- including payment processing and fulfillment and shipping solutions -- to help them run their businesses more efficiently. But just how big an opportunity is e-commerce

This industry will expand at a compound annual growth rate of 14.7% through 2027, according to the research firm Grand View Research. And Shopify itself estimates its total addressable market to be at least $153 billion.

Small boxes on a laptop's keyboard.

Image source: Getty Images.

For context, the company's trailing-12-month revenue stands at $2.93 billion, which, by the way, represents an 86% year over year increase. In short, the company's prospects are exciting. All that's left for Shopify is to execute its long-term plan. That is, the company needs to continue attracting new merchants while retaining the ones it already has. Shopify is heavily investing in marketing efforts -- both in the U.S. and abroad -- to get more sellers onto its platform.

Given the convenience of its services, and considering the rapid growth in the number of merchants on its website (1.7 million at the end of 2020, up from 820,000 at the end of 2018), investors have every reason to think these efforts will be successful. And Shopify can keep most of its clients thanks to its platform's high switching costs. 

Building an online storefront from scratch takes considerable time and effort. Once having gone through this trouble, sellers on Shopify are unlikely to switch to a competing e-commerce platform, since that would require them to start the process all over.

Thanks to the massive opportunities at its disposal, and the competitive advantage it is building, Shopify's stock looks like a strong buy, especially if you can scoop up shares when they dip during the next market crash.