Space is a type of scarcity and is concerned with the size of things. Data shuttles across networks and into machines where it is stored ephemerally in RAM and possibly saved to permanent storage in the form of hard or solid-state-drives (HDDs, SSDs). As proponents of efficiency, we are interested in only using the space necessary to do a given task, and part of this is using the available space in the most efficient manner, and only moving data when it is prudent to do so.

Due to the high-level nature of the JavaScript language and the applications it usually builds, we rarely have to think about ephemeral RAM usage or permanent storage. However, JavaScript has gained significant ground as a legitimate language in performance-sensitive environments, such as database infrastructure and HTTP middleware, so these concerns are far more relevant nowadays.

Furthermore, the demands of client-side applications, both in the browser and within native environments, has drastically increased. The complexity of these applications means that we must always be on our toes, thinking about how to optimize memory and bandwidth usage on servers, user devices, and across increasingly layered networks. The bandwidth we absorb in a web application will have a direct effect on the time the user has to wait for the application to become usable.

Time and space efficiency are inextricably linked with both directly affecting the other. The overarching theme of efficiency is about only doing what's necessary, avoiding waste, and being thrifty with the resources you have available.