can either accept another function as an argument or return a function.
are functions that have no side effects. Side effects are actions a function may perform that are not solely contained within the function itself.
allows us to write smaller, more concise algorithms and to operate by looking only at the inputs to our functions.
Immutable variables, once set, cannot be changed.
Nonstrict (Lazy) evaluation
allow us to have variables that have not been computed yet.
Strict evaluations-assigning a variable as soon as it is defined-are what we are used to
Nonstrict means that we can have a variable that does not get assigned (computed) until the first time it is referenced.
are evaluable pieces of code that have a return value.
Each line of code should be considered a statement, meaning there are very few side effects within the application itself.
allows us to better type-check and extract elements from an object, making for simpler and more concise statements with less need for variable definitions.
Closures are much like lambdas, except they reference variables outside the scope of the function.
In the simplest explanation, the body references a variable that doesn't exist in either the body or the parameter list.
Lambda functions are unnamed functions that contain a parameter list, a body, and a return.
A function becomes "higher order" if it accepts or returns a function.
Output Depends on Input
When we don't return the result of our execution but rather mutate another external (i.e., not contained within the function scope) object, we call this a side effect.
Pure functions are functions that have no side effects and always perform the same computation,resulting in the same output, given a set of inputs.
Really, you want to make a function pure whenever possible; it makes the function much more testable and improves understandability from a troubleshooting perspective.