Browser-based research enables flexible, location-independent data collection, with access to larger, more representative samples. With lab.js, a free and open-source experiment builder, stimuli can be designed and combined into studies using a graphical interface, without writing code. Experiments constructed with lab.js can be run both online and in-laboratory, hosted via multiple public data collection services as well as on researchers’ own web servers. Through empirical validation studies, we show that lab.js keeps and measures presentation and response times with high accuracy and precision across platforms. The software is also built to facilitate open, cumulative science: Studies can be shared in an editable format, archived, adapted and re-used, enabling effortless, transparent replications. The software is provided free of charge under an open-source license; further information, extensive documentation and teaching materials are available from https://lab.js.org
