Array Methods Flashcards
Check whether a thing is an array
SIMPLE LANGUAGE:
Is the argument an Array?
Array.isArray(input)
Iterate over each array element. The return from a callback function is passed to the next iteration.
SIMPLE LANGUAGE:
Analyze all array elements, allow a variable to pass from one iteration to the next
myArray.reduce(callback function(acc, arg) => {
acc += arg}, accStart
)
ex.
> let arr = [2, 3, 5, 7]
> arr.reduce((accumulator, element) => accumulator + element, 0)
let apple = [1, 2, 3, 4, 5, 6]
console.log(apple.reduce((acc, value) =>
[(acc[0] + value), (acc[1] - value)], [0,0] ));
logs: [ 21, -21 ]
Analyze all of an array’s elements, returns the elements in a new array based on whether a function for each element is true.
SIMPLE LANGUAGE:
Create a new array from old array filtered based on some condition.
myArray.filter(element => if true return element as part of new array)
Return boolean if a string contains a specified substring
or
if an array element matches the entirety of the argument
SIMPLE LANGUAGE:
Does the array contain this entry?
Does the string contain this substring?
myStringOrArray.includes(‘value’)
SIMPLE LANGUAGE:
Cut (mutate) an array and return the cut section
myArray.splice(start index, number of indexes)
SIMPLE LANGUAGE:
Add argument to the end of an array and return the new array length
myArray.push(new element);
SIMPLE LANGUAGE:
Add argument to the beginning of an array and return the new array length
myArray.unshift(new element);
Return a combined array or string from the argument.
Returns string if method is used on a string.
Returns array if method is used on an array.
SIMPLE LANGUAGE:
Add string arguments to the end of a string
Add arrays, or other arguments to the end of an array.
myStringorArray.concat( , )
does not mutate.
Remove the last element from an array and return the element
myArray.pop();
Remove the first element from an array and return the element
myArray.shift();
Return a copied section from an array (or string)
What happens if the first argument equals the second?
What happens if there is no second argument?
myArray.slice(first index, last index exclusive)
If first index = last index, an empty array is returned
It returns an array/string from that index to the last
Reorganize an array’s elements in place (mutating). It also returns the organized array
No arg defaults to string comparison
myArray.sort( )
Reverse an array’s elements in place (mutating).
It also returns the reversed array
Array.prototype.reverse( )
Return the Array with nested arrays unnested.
can choose level of unesting
myArray.flat( number of levels to unnest)
Return combined array elements into a string with argument in between
myArray.join(‘ ‘)
What’s the difference between Array.from and .map() ?
Array.from() works with array-like objects like strings
Array.from() is meant to create arrays from array like objects and can take a callback function
.map() is for arrays
.map() is meant to map a new array off an existing one using a callback function. It has more functionality
.map() is 15 times faster
Return true if at least 1 array element returns true from a callback function
SIMPLE LANGUAGE:
Do some of the array element follow this pattern?
myArray.some( )
Return true if all array elements returns true from a callback function
SIMPLE LANGUAGE:
Does every array element follow this pattern?
myArray.every()
What’s the difference between .some() and .includes()?
.includes() is just a simple comparison of the argument and array values.
.some() lets you use a callback function to test whatever logic you want.
Returns the first element that returns true from a callback function.
SIMPLE LANGUAGE:
What’s the first element in the array that follows this pattern?
myArray.find( callback fuction)
Replace an array’s elements between certain indexes with a SINGLE entry
Returns the changed array
SIMPLE LANGUAGE
Replace each consecutive elements with a 1 argument.
myArray.fill(replacement, start ind, end exclusive index)
mutates
[1, 2, 3, 4, 5, 6, 7, 8].fill(“apple”, 4, 6)
Expected output:
[ 1, 2, 3, 4, ‘apple’, ‘apple’, 7, 8 ]
Return the Array elements based on a callback function and with first-level nested arrays unnested.
Does it map first or flat first?
SIMPLE LANGUAGE
Map an array based on callback and flat the result
myArray.flatMap( callback function)
It maps then flats.
What is returned from:
[1,2].concat([3,4]);
[1,2].concat([[3,4]]);
‘string’.concat(342432)
output=> [1,2,3,4]
output=> [1,2,[3,4]]
output=> “string342432”
What is returned from:
[1, 2, 3].pop().shift()
an error
Because it is evaluated from left to right
[1, 2, 3].pop() evaluates to 3
and 3.shift() give an error
What are the similarities and difference between .substring() and .slice()?
What happens when only 1 argument?
If start index > end index, what happens?
What do indexes start and finish?
what do they work on?
What happens when second argument larger than length?
how do they repond to negative numbers or NaN?
Similarities
- Both go from start index (inc) to end index (exc)
- If end index is excluded they go to the end
- If end index > length, end index = length;
Differences
- slice works with strings and arrays, substring only works on strings.
- If start index > end index, .slice() will return empty string/ array. .substring() will swap the indexes.
- both act different with negative arguments and NaN
https: //stackoverflow.com/questions/2243824/what-is-the-difference-between-string-slice-and-string-substring
What is logged to console from this:
let apple = [1, 2, 3, 4, 5, 6]
console.log(apple.reduce((acc, value) =>
[(acc[0] + value), (acc[1] - value)], [0,0] ));
Why?
logs: [ 21, -21 ]
The accumulator is set to: [0,0]
The callback function returns 2 elements in an array
(acc[0] + value) and (acc[1] - value)
The first accumulator element plus the value
The second accumulator element minus the value
What is the difference between each of these (what do they do and return?): Array.prototype.includes() Array.prototype.some( ) Array.prototype.every() Array.prototype.filter() Array.prototype.find() Array.prototype.indexOf()
.includes() - returns boolean based on whether the argument is within the array
.some( ) - iterates over each element and returns boolean if at least 1 iteration returns true based on a callback function
.every() - iterates over each element and returns boolean if all iterations return true based on a callback function
.filter() - iterates over each element and returns an array based on whether the elements evaluate to true based on a callback function
.find() - returns the value of the first element that returns true from a callback function
.indexOf() returns the first index of an element that matches the argument
What’s the difference between .map() and .forEach() ?
2 differences
.forEach does a function for each element in an array.
.map() returns an array based on a callback function return.
- .forEach() returns undefined (and can only be used at the end of a chain usefully)
.map() returns the new mapped array (and is therefore chainable from)
2..map() does not mutate and returns an array
.forEach() just allows a callback function iterating over array elements
What does
Array.prototype.includes()
do?
and/or return?
Determines whether an array includes a certain value among its entries, returning true or false as appropriate.
includes(searchElement)
includes(searchElement, fromIndex)
(There is also a String.prototype.includes())
What does
Array.isArray()
do?
and/or return?
Returns boolean based on whether the passed value is an Array.
Array.isArray(value)
What does
Array.prototype.reduce()
do?
and/or return?
Executes a user-supplied “reducer” callback function on each element of the array, in order, passing in the return value from the calculation on the preceding element. The final result of running the reducer across all elements of the array is a single value.
// Arrow function
reduce((previousValue, currentValue) => { /* ... */ } )
reduce((previousValue, currentValue, currentIndex) => { /* ... */ } )
reduce((previousValue, currentValue, currentIndex, array) => { /* ... */ } )
reduce((previousValue, currentValue, currentIndex, array) => { /* ... */ }, initialValue)
// Callback function
reduce(callbackFn)
reduce(callbackFn, initialValue)
// Inline callback function
reduce(function(previousValue, currentValue) { /* ... */ })
reduce(function(previousValue, currentValue, currentIndex) { /* ... */ })
reduce(function(previousValue, currentValue, currentIndex, array) { /* ... */ })
reduce(function(previousValue, currentValue, currentIndex, array) { /* ... */ }, initialValue)
What does
Array.prototype.filter()
do?
and/or return?
Creates a new array with all elements that pass the test implemented by the provided function.
// Arrow function
filter((element) => { /* ... */ } )
filter((element, index) => { /* ... */ } )
filter((element, index, array) => { /* ... */ } )
// Callback function
filter(callbackFn)
filter(callbackFn, thisArg)
// Inline callback function
filter(function(element) { /* ... */ })
filter(function(element, index) { /* ... */ })
filter(function(element, index, array){ /* ... */ })
filter(function(element, index, array) { /* ... */ }, thisArg)
What does
Array.prototype.splice()
do?
and/or return?
changes the contents of an array by removing or replacing existing elements and/or adding new elements in place
Returns the cut section
splice(start)
splice(start, deleteCount)
splice(start, deleteCount, item1)
splice(start, deleteCount, item1, item2, itemN)
What does
Array.prototype.push()
do?
and/or return?
adds one or more elements to the end of an array and returns the new length of the array.
push(element0)
push(element0, element1)
push(element0, element1, /* … ,*/ elementN)
What does
Array.prototype.unshift()
do?
and/or return?
adds one or more elements to the beginning of an array and returns the new length of the array.
unshift(element0)
unshift(element0, element1)
unshift(element0, element1, /* … ,*/ elementN)
What does
Array.prototype.concat()
do?
and/or return?
The concat() method combines two or more arrays AND/OR adds the arguments to the target array. This method does not change the existing arrays, but instead returns a new array.
concat()
concat(value0)
concat(value0, value1)
concat(value0, value1, … , valueN)
(There is also a String.prototype.concat())
What does
Array.prototype.pop()
do?
and/or return?
removes the last element from an array and returns that element. This method changes the length of the array.
pop()
What does
Array.prototype.shift()
do?
and/or return?
removes the first element from an array and returns that removed element. This method changes the length of the array.
shift()
What does
Array.prototype.slice()
do?
and/or return?
returns a shallow copy of a portion of an array into a new array object selected from start to end (end not included) where start and end represent the index of items in that array. The original array will not be modified.
slice()
slice(start)
slice(start, end)
What does
Array.prototype.sort()
do?
and/or return?
How does it’s algorithm work?
sorts the elements of an array in place and returns the sorted array. The default sort order is ascending, built upon converting the elements into strings, then comparing their sequences of UTF-16 code units values.
Mutates
// Functionless sort()
// Arrow function
sort((firstEl, secondEl) => { /* ... */ } )
// Compare function sort(compareFn)
// Inline compare function
sort(function compareFn(firstEl, secondEl) { /* ... */ })
If
[1, -2, 15, 2, 0, 8].sort(function(a, b) {
});
It will put a number in a
It will put a number in b
If the return of the function is positive, it will put b after a.
What does
Array.prototype.reverse()
do?
and/or return?
reverses an array in place. The first array element becomes the last, and the last array element becomes the first.
Mutates
Returns the reversed array
reverse()
What does
Array.prototype.flat()
do?
and/or return?
creates a new array with all sub-array elements concatenated into it recursively up to the specified depth.
flat()
flat(depth)
What does
Array.prototype.join()
do?
and/or return?
returns a new string by concatenating all of the elements in an array (or an array-like object), separated by commas or a specified separator string. If the array has only one item, then that item will be returned without using the separator.
join()
join(separator)
What does
Array.from()
do?
and/or return?
static method creates a new, shallow-copied Array instance from an array-like or iterable object.
// Arrow function
Array.from(arrayLike, (element) => { /* ... */ } )
Array.from(arrayLike, (element, index) => { /* ... */ } )
// Mapping function Array.from(arrayLike, mapFn) Array.from(arrayLike, mapFn, thisArg)
// Inline mapping function
Array.from(arrayLike, function mapFn(element) { /* ... */ })
Array.from(arrayLike, function mapFn(element, index) { /* ... */ })
Array.from(arrayLike, function mapFn(element) { /* ... */ }, thisArg)
Array.from(arrayLike, function mapFn(element, index) { /* ... */ }, thisArg)
What does
Array.prototype.map()
do?
and/or return?
creates a new array populated with the results of calling a provided function on every element in the calling array.
// Arrow function
map((element) => { /* ... */ })
map((element, index) => { /* ... */ })
map((element, index, array) => { /* ... */ })
// Callback function map(callbackFn) map(callbackFn, thisArg)
// Inline callback function
map(function(element) { /* ... */ })
map(function(element, index) { /* ... */ })
map(function(element, index, array){ /* ... */ })
map(function(element, index, array) { /* ... */ }, thisArg)
What does
Array.prototype.some()
do?
and/or return?
“Are there some values in the array for which the given callback returns a truthy value?
tests whether at least one element in the array passes the test implemented by the provided function. It returns true if, in the array, it finds an element for which the provided function returns true; otherwise it returns false. It doesn’t modify the array.
// Arrow function
some((element) => { /* ... */ } )
some((element, index) => { /* ... */ } )
some((element, index, array) => { /* ... */ } )
// Callback function
some(callbackFn)
some(callbackFn, thisArg)
// Inline callback function
some(function(element) { /* ... */ })
some(function(element, index) { /* ... */ })
some(function(element, index, array){ /* ... */ })
some(function(element, index, array) { /* ... */ }, thisArg)
What does
Array.prototype.every()
do?
and/or return?
The every() method tests whether all elements in the array pass the test implemented by the provided function. It returns a Boolean value.
// Arrow function
every((element) => { /* ... */ } )
every((element, index) => { /* ... */ } )
every((element, index, array) => { /* ... */ } )
// Callback function
every(callbackFn)
every(callbackFn, thisArg)
// Inline callback function
every(function(element) { /* ... */ })
every(function(element, index) { /* ... */ })
every(function(element, index, array){ /* ... */ })
every(function(element, index, array) { /* ... */ }, thisArg)
What does
Array.prototype.find()
do? and/or return?
The find() method returns the value of the first element in the provided array that satisfies the provided testing function. If no values satisfy the testing function, undefined is returned.
// Arrow function
find((element) => { /* ... */ } )
find((element, index) => { /* ... */ } )
find((element, index, array) => { /* ... */ } )
// Callback function
find(callbackFn)
find(callbackFn, thisArg)
// Inline callback function
find(function(element) { /* ... */ })
find(function(element, index) { /* ... */ })
find(function(element, index, array){ /* ... */ })
find(function(element, index, array) { /* ... */ }, thisArg)
What does
Array.prototype.fill()
do?
and/or return?
The fill() method changes all elements in an array to a static value, from a start index (default 0) to an end index (default array.length). It returns the modified array.
Mutates
fill(value)
fill(value, start)
fill(value, start, end)
What does
Array.prototype.flatMap()
do?
and/or return?
The flatMap() method returns a new array formed by applying a given callback function to each element of the array, and then flattening the result by one level. It is identical to a map() followed by a flat() of depth 1, but slightly more efficient than calling those two methods separately.
// Arrow function
flatMap((currentValue) => { /* ... */ } )
flatMap((currentValue, index) => { /* ... */ } )
flatMap((currentValue, index, array) => { /* ... */ } )
// Callback function
flatMap(callbackFn)
flatMap(callbackFn, thisArg)
// Inline callback function
flatMap(function(currentValue) { /* ... */ })
flatMap(function(currentValue, index) { /* ... */ })
flatMap(function(currentValue, index, array){ /* ... */ })
flatMap(function(currentValue, index, array) { /* ... */ }, thisArg)
What does
Array.prototype.indexOf()
do?
and/or return?
returns the first index at which a given element can be found in the array, or -1 if it is not present.
indexOf(searchElement, fromIndex)
(There is also an indexof for strings)
Which of these take a callback function: Array.prototype.includes() Array.prototype.some( ) Array.prototype.every() Array.prototype.filter() Array.prototype.find() Array.prototype.indexOf()
.some()
.every()
.filter()
.find()
Return the first index in an array that matches the given argument
Can also start from a given index
Array.prototype.indexOf()
What’s the difference between .map() and .filter() and Array.from()
.filter() creates a new array based on whether the elements return boolean on a callback function
.map creates a new array based on the return of a callback function
Array.from() is for array like object. It is slower than .map. but otherwise functions similarly
Return the elements of an array as a string separated by commas
2 ways
Array.prototype.toString()
String(arr)
What does:
Array.prototype.toString()
do and/or return?
Return the elements of an array as a string separated by commas
t or f. Why?
alert( 0 == [0] );
alert( ‘0’ == [0] );
alert( [0] == [0] );
alert( 0 == [] );
alert( ‘’ == [] );
alert(‘0’ == [] );
false- non empty arrays compared to primitives will convert to strings, ‘0’
true-
false- these arrays are unique objects
true- empty arrays will convert to 0 when compared to numbers
true- empty arrays will convert to ‘’ when compared to strings
false- ‘0’ == ‘’
Insert elements into the middle of an array
arr.splice( 2, 0, ‘insert this’, ‘and this’)
It wil insert it AT position 2. It will push the old position 2 to position 3
What happens when you use negative indexes with array methods?
How about as an array index? what is returned from each of these?
arr[-1]
arr[-1] = 1;
arr[-1]
It starts from the end of the array.
undefined, looking for a string ‘-1’ as a key likely
makes the -1 a key as a regular object property
returns 1 as the object value
it will be listed in the array as: ‘-1’: 1
Returns the index of the first element in the array that satisfies the provided testing function. Otherwise, it returns -1, indicating that no element passed the test.
Array.prototype.findIndex()
What does:
Array.prototype.findIndex()
do and/or return?
Returns the index of the first element in the array that satisfies the provided testing function. Otherwise, it returns -1, indicating that no element passed the test.
How is the array altered?
[9, 21, 1].sort()
[1, 21, 9] The elements are sorted as strings. How do you make it not do this? (scroll down) . . . . . .
. . . . . . . .
.
.
.
To make it not do this:
function compare(a, b) {
if (a > b) return 1; // if the first value is greater than the second
if (a == b) return 0; // if values are equal
if (a < b) return -1; // if the first value is less than the second
}
[9, 21, 1].sort(compare() )
// Functionless sort()
// Arrow function
sort((firstEl, secondEl) => { /* ... */ } )
// Compare function sort(compareFn)
// Inline compare function
sort(function compareFn(firstEl, secondEl) { /* ... */ })
What does
Array.prototype.reduceRight()
do?
and/or return?
same as reduce but starts on the end of the array
Do a reduce method but start from the end
Array.prototype.reduceRight()
What is “thisArg”?
A parameter in many methods that sets the value of this.
thisArg refers to context which callback should be called, basically it is what this refers to inside callback. For example:
var myObject = { name: ‘myObject’ };
[1,2].forEach(function(item) {
console.log(item); // 1, 2
console.log(this === myObject); // true
}, myObject)
