Amjad Masad built software developer tools for Facebook's thousands of engineers for two years. Then he decided he needed to leave the company to build the online developer tool he most wanted.

"At some point I looked back at the space of online dev tools, and I found that no one had really built the start-up that I wanted to build," he said. "I had to quit my job at Facebook to pursue it."

That start-up is called Repl.it, and its goal is to make it possible for anyone to quickly write and ship code from their web browsers.

Typically when software engineers want to build an app, they use a text editor to write code, save it in a second repository service like GitHub, then run it on a server or a public cloud such as Amazon Web Services.

"We've eliminated all those steps, and you just open a new browser tab, go to Repl.it and you have all these things in a single environment," Masad said.

That simplicity has helped Repl.it grow to 1 million monthly active engineers since launching in early 2016. The San Francisco start-up on Monday announced a $4.5 million initial funding round led by Andreessen Horowitz.

Masad co-founded the company alongside his wife and designer Haya Odeh. His interest in online-based developer tools began when Masad built a similar tool for Codecademy, an online coding school, that allowed software engineering students to write code in their web browsers.

The start-up, which has a staff of six, plans to use the capital to ramp up hiring and begin commercializing the product, Masad said.

"The really interesting thing with Repl.it is that their audience is super wide," said Andrew Chen, Andreessen Horowitz general partner. "It includes kids and students who are programming for the first time."

Andreessen Horowitz has invested in several developer tool companies in the past, including Stack Overflow and GitHub, which was acquired in June by Microsoft for $7.5 billion. The firm is betting that the market's shortage of software engineers and Repl.it's ease of use will bring it broad appeal.

"These are often programmers and developers building websites who otherwise wouldn't be able to," Chen said. "Repl.it makes it so easy that this audience can just go and do that."

The funding round brings Repl.it's total funding to $5.8 million. Others joining Andreessen Horowitz include Bloomberg Beta, Reach Capital, Y Combinator and the investor Paul Graham.