With the adoption of Agile development and DevOps culture, a story/feature is not complete without testing, and rather than being reactive by looking for bugs after development, full-stack QEs will focus on proactively looking for defects throughout the planning, design and development stages. When full-stack QEs work within a developer’s technical stack (as in the example, working within the JS stack as the developer), they can be Agile, prevent bugs, and increase engineering productivity. Full-stack QEs treat testing as defect prevention than defect detection.
Reference: http://syscolabs.lk/blog/