Note: Eloquent JavaScript was recommended to me by Leon from #100Devs. While I’ve been participating in a JavaScript study using The Modern JavaScript Tutorial, I had an itch to also read Eloquent JavaScript as a refresher.

Eloquent JavaScript TIL - Chapters 4 and 5.

Chapter 4 - Data Structures

[] brackets can be used to access the n-th index of an array, as well as a property of an object (note: if you are putting the name of the property into the [], you have to wrap the property name with quotes).
If you omit the quotes around the square bracket property name, this name is evaluated to get the property name. Therefore, (myUser[name] and myUser['name'] are not the same).
Object properties can be accessed using dot notation as well (Math.max). You can’t use the dot notation with numbers (need bracket notation).
null and undefined do not have properties
Accessing an object’s non-existent property results in undefined

Primitive values such as strings, numbers, and Booleans are all immutable - you cannot change values of those types; they cannot be altered. The variable bound to a primitive value can be reassigned to a new value, but the existing value itself (e.g. the number 120, or string ‘hello’) cannot be changed in ways like mutable data structures can.
Variables with objects stored aren’t actually storing the object, but rather their reference value (e.g. kind of like a street address).
Two different variables could hold the same reference value for the identical object, so if you
You can still add properties to const obj = {} even when you created the object using const keyword because the reference value pointing to the object remains the same even if you add properties.

Instead of using the classic for loop you can also use for ... of to loop through arrays (as well as strings - any iterable structure).

Array.slice() creates a new array, whereas Array.splice() methods the array in place.
String.trim() removes whitespace (including spaces, newlines, tabs from the start and end of a string)
String.padStart(desired length, padding character) checks whether a given string is equivalent to desired length parameter and if not pads more characters using padding character.

I always get these confused because they use the same three dot notation.
Rest parameter - when you set a parameter during function declaration/definition to have ... with a variable name, this is the rest parameter. You are putting all the parameters together into an array.
Spread operator - used when passing an argument during a function call. This spreads the elements of an array into each element.
You can use spread operator to spread another array or object into a new array/object.

Abstraction: hides the details (the “how”) of the program and focuses more on only revealing the “what”.
Higher-order functions: functions that operate on other functions by either a) taking them as arguments, or b) returning functions
Higher-order functions can take another function as its argument and modify its behavior (wrapper)
Array.reduce(array, combine, start) - you can leave off the start parameter if the array you’re passing has at least one element.