What is scope?

The scope of a variable is region in which that variable is accessible. In Javascript, there are two regions: global and local.
Variables with a global scope are accessible (and can be mutable, depending on their declaration) by all functions. This can
even include functions in other files.

Variables with a local scope, by contrast, are declared within a function, and are only accessible within that function.

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Why are global variables to be avoided?

A variable that can be accessed anywhere is prone to unwanted side-effects. Functions, generally, ought to be determinate. That
is, if the input to a function remains the same, its output should as well. If a function consumes a global variable--one which
can be used and changed anywhere--its output could be different given the same input on different occassions, if that global
variable's value changed.

While this can sometimes be desirable, it can lead to unpredictable and unwanted results. This is especially true as a program
grows in size and complexity. It is therefore good practice to give a variable the smallest scope needed to do its job.

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Explain Javascript's strict mode

Javascript's strict mode raises an exception when a variable is declared without 'let' or 'const'. The purpose is to limit
unwanted-side effects of manipulating global variables.

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What are side-effects? What is a pure function?

Side-effects are changes a function makes outside of itself--changing the value of a global variable, for example. A pure
function is is determinate (there can be no change in output without a change in input) and has no side-effects.