Declarative programming is when you tell what you want. Imperative is when you say how to get what you want.

Closure is when a function is able to remember and access its lexical scope even when that function is executing outside its lexical scope. Source: https://astronautweb.co/javascript-lexical-scope/

In a nested group of functions, the inner functions have access to the variables and other resources of their parent scope. This means that the child functions are lexically bound to the execution context of their parents. Lexical scope is sometimes also referred to as static scope. Source: https://stackoverflow.com/a/53062093

They verify if a proposed functionality or a bug fix works as expected. There is a difference between sanity and smoke tests.

Smoke tests verify if the most important features works as usual to prevent wasting QA team time. There is a difference between smoke and sanity tests.

All non-trivial abstractions, to some degree, are leaky. Joel Spolsky To be able to use reliable software, we need to learn abstraction’s underlying details anyway. Source: https://en.wikipedia.org/wiki/Leaky_abstraction

Selectivity is the number of unique values produced by an operation (e.g. an index scan or filter), relative to the total number of rows. The higher the selectivity, the more likely PostgreSQL is able to use an index. Source: https://docs.gitlab.com/ee/development/understanding_explain_plans.html#cardinality-and-selectivity

The number of unique values in a column in a table.

Mean Time Between Failures experienced by end-users. The higher the time, the more reliable the system.

We make the biggest decisions when we have the least knowledge. Note to myself: Try to chose flexible solutions and be open to adjust them when you know more about the project.