Being proficient in your tech stack helps your output sure but it's only one part (and a small one IMO) of the equation for creating something like esbuild. The

Being proficient in your tech stack helps your output sure but it's only one part (and a small one IMO) of the equation for creating something like esbuild. The author of esbuild surely has a strong knowledge of computer architecture, data structure and algorithms, operating systems and some math.