Arrays Flashcards
How do we create an array?
let arrayName = []
What can we store in an array?
- Any valid data type
- objects
- arrays
- functions
How do we find the value of an item in an array?
we query the array with arrayName[n]
n is the index of the item in the array
What happens if we query an array with an index larger than the array holds
we get ‘undefined’ as the result
Can we get items from the end of an array?
Yes, by using negative indices we can query from the end of an array.
How could target the end of an array?
arrayName[arrayName.length - 1]
How can we assign a value to an item in an array
arrayName[n] = someValue
will create or reassign the value of an item in an array
if you insert a item value deep into an array, what will the intervening values be set to?
undefined
(or empty in google chrome)
What method could we use to find the index of a value in an array?
We could use the .indexOf method to find the index of a value.
Describe the indexOf method syntax on an array
arrayName.indexOf(value)
What data types will indexOf work on?
indexOf works on primitive data types
If a value is not present in an array, what does indexOf return
indexOf will return -1 if a value does not exist in an array.
How can we add a value to the front of an array?
we can use the unshift method to add a value to the front of an array.
How can we add a value to the end of an array?
We can use the push method to add a value to the end of an array.
What are two methods used for adding values to an array
- .unshift - to the front of an array
- .push - to the end of an array
How can we remove a value from the start of an array?
We can use the .shift method to remove a value from the start of an array
What is the syntax for using the shift method on an array
arrayName.shift()
What method can we use to remove a value from the end of an array
We can use the pop method to remove a value from the end of an array
Name two methods we can use to remove values from an array
- .shift - remove from front of array
- .pop - remove from end of array
What does the splice method allow us to do?
It allows us to add or remove values from any position in an array
What is the splice method syntax on an array?
arrayName.splice(index, deleteCount, itemsToAdd)
When using the splice method, where does the editing begin?
It begins with the index value given as the first argument in the method.
If we want to only add, and not delete, items using the splice method, how do we do so?
We set the the deleteCount argument, the second argument to the splice method, to zero.
.splice(index, 0, itemsToAdd)
If we are removing items from an array using splice, which index will it remove first
splice will remove the length starting with the index passed as the first argument.
If we are adding items to an array using splice, which value will be the first to change.
the splice method begins editing at the index given as its first argument, so:
- splice(0,0,1) makes index 0 = 1
- splice(3,0,2) makes index 3 = 2
If we are adding items to an array using splice, which value will be the first to change.
the splice method begins editing at the index given as its first argument, so:
- splice(0,0,1) makes index 0 = 1
- splice(3,0,2) makes index 3 = 2
How do we use the slice method on an array?
The slice method copies a section of an array, without changing the original
We generally save this new array to a variable
How do the splice and slice methods on arrays differ
Splice modifies the original array
Slice copies a slice of the original, but does not modify the original array
Give the syntax for the slice method on arrays
arrayName.slice(start, end)
start = includes start index
end = excludes end index
Given:
array = [1, 2, 3, 4, 5, 6]
newarray = array.slice(1, 4)
What does newarray contain?
newarray = [2, 3, 4]
Do we need to pass parameters to the shift method.
No, the shift method remove only the first element in an Array
If an array is mutated in a function, does the mutation persist outside the function scope?
Yes, if the mutation was destructive, then the mutation persists.
What do we mean by passed-by-reference?
passed-by-reference refers to the fact that when we pass an array into a function, we are passing a reference to that array in memory.
How do we use the .find method in JavaScript?
The .find method is used to check an array of objects for the presence of an object with a specific condition.
How many arguments are typically used in the .find method?
We typically only use one argument in the .find method, the item argument (the others are index, array
What is the syntax for using a .find method on an array of objects in JavaScript?
array.find(item => item.id == 1);
This would return the object with the presence of id: 1
If we saved the result to a variable, we could then access other key:value pairs within the object
let user = users.find(item => item.id == 1)
console.log(user.name) - would log the value of the name key from the object with id: 1
When does the method .find stop iterating over an array?
.find stops iterating over an array when the first truthy value is found.
If nothing is found it returns undefined.
What method should we use on an array if we want all of a certain value returned?
We should use the .filter method on an array to return all the occurrences of a certain value.
What is the syntax of a .filter method call?
let results = array.filter(item => item > 10)
results would then be an array with any value > 10
Can we use the .filter method on an array of objects?
Yes, .filter can be used on an array of objects to create an array of all objects matching certain criteria.
Give the syntax of using the .filter method on an array of objects.
let results = arr.filter(item => item.id < 3)
for(let result of results){
console.log(result.name)
}
How is the .sort method used on arrays?
The .sort method with sort an array alphabetically (even if the contents are intergers), unless we provide a function as an argument to .sort
What is a typical .sort syntax used to sort items numerically?
arr.sort((a,b) => a - b)
when a - b produces a positive result the order needs to be reversed (i.e. a > b)
for this reason, to sort in reverse we use b - a, when this produces a positive result the order is kept (i.e. b > a)
What method might we pass to the function inside the .sort method to sort strings alphabetically?
We would use the localeCompare method because, unlike sort a - b, it compares the letters alphabetically instead of based on their character code value.
arr.sort((a,b) => a.localeCompare(b)
Instead of sorting using b - a in our .sort function, what might we do to reverse an array?
We could use the .reverse method to sort from higher to lower (in descending order)
Why is it good practice to include the initial value when using the .reduce method?
It is good practice, because if the array is empty the .reduce method will throw an error.
What is the syntax of a reduce method call?
let arrSum = arr.reduce((sum, current) => sum + current, 0)
Where the 0 is the initial value to start with.
What does the method .reduceRight do for us in JavaScript?
.reduceRight begins it’s iteration from the right side of the array
What does the syntax Array.isArray(x) do for us in JavaScript and why would we use it?
Array.isArray(x) checks to see if x is an array, and returns true if it is.
We use it to determine if a data structure is an array or not, as in JavaScript, arrays are object-based data-structures.
