JavaScript’s first working prototype was built in about ten days. Brendan Eich, who created the language at Netscape, wrote the initial version, code-named Mocha, under heavy deadline pressure to meet a feasibility demonstration for the Netscape Navigator 2.0 release.
The most authoritative account is the history “JavaScript: The First 20 Years,” which Eich co-wrote with Allen Wirfs-Brock. It states that Eich “prototyped the first Mocha implementation in ten contiguous days in May, 1995,” and notes that while no record of the exact dates survives, Eich believes it was May 6 to 15. At the end of those ten days the prototype was demonstrated to the full Netscape engineering staff.
The ten-day figure is one of the most repeated facts in the history of programming. It is often cited to explain both how quickly the web gained a built-in scripting language and why that language carried some rough edges from the start. Eich has confirmed the story in his own firsthand writing, making it a rare widely-repeated claim that comes straight from its source.