This essay attempts a philological, meaning a both technically and socially attentive historical study of an individual computer programming language, JavaScript. From its introduction, JavaScript’s reception by software developers, and its importance in web development as we now understand it, was structured by a continuous negotiation of expertise. I use the term “improvised expertise” to describe both conditions for and effects of the unanticipated development of JavaScript, originally designed for casual and inexpert coders, into a complex of technical artifacts and practices whose range and complexity of use has today propelled it into domains previously dominated by other, often older and more prestigious languages. “Improvised expert...