Arrays
Can use the new keyword, but much easier to just assign an array to a variable:
arr1 = new Array("purple", "green", "yellow");
arr2 = new Array(10); // creates an array with 10 undefined values
bestAr = ["black", "orange", "pink"];
A single array can hold elements of a different type:
let arr = ["hi there", 5, true];
console.log(typeof arr[0]); // string
console.log(typeof arr[1]); // number
console.log(typeof arr[2]); // boolean
You can change the values of elements of a const array, but you can not reassign it to a new array:
// let array
let lArr = [1, 2, 3, 4, 5]
lArr = ["one", "two", "three", "four", "five"]
lArr
(5) ['one', 'two', 'three', 'four', 'five']
// cannot reassign const array
const cArr = [1, 2, 3, 4, 5]
cArr = ["one", "two", "three", "four", "five"]
VM1349:1 Uncaught TypeError: Assignment to constant variable.
at <anonymous>:1:6
Properties
fruit = [“apple”, “banana”, “pear”] (3) [‘apple’, ‘banana’, ‘pear’] fruit.length 3
arr.length // length is a technically a property
// push adds elements to the end of the array and
// it returns the new length of the array
array.push(val)
let lenArray = array.push(val)
// splice adds elements at a specific index, and optionally deletes
// any elements with the second argument
// splice(start, numDel, els...)
> let fruits = ['apple', 'banana', 'orange']
> fruits.splice(2, 0, 'kiwi', 'grape')
> fruits
[ 'apple', 'banana', 'kiwi', 'grape', 'orange' ]
> fruits.splice(2, 1, 'pear')
[ 'kiwi' ]
> fruits
[ 'apple', 'banana', 'pear', 'grape', 'orange' ]
// Use splice and only the first two arguments to delete a range of elements from an array.
// Both arguments are inclusive.
> nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
> nums.splice(1, 5)
[ 1, 2, 3, 4, 5 ]
> nums
[ 0, 6, 7, 8, 9 ]
// concat combines arrays
> let a = [1, 2, 3]
> let b = [4, 5, 6]
> let c = a.concat(b)
> c
[ 1, 2, 3, 4, 5, 6 ]
// .pop() deletes the last element from the array and returns it
> c.pop()
6
> c
[ 1, 2, 3, 4, 5 ]
> let p = c.pop()
> p
5
> c
[ 1, 2, 3, 4 ]
// .shift() deletes the first element from the array
[ 1, 2, 3, 4, 5, 6 ]
> c.shift()
1
> s = c.shift()
2
> s
2
> c
[ 3, 4 ]
// the delete operator removes an item from an array but does not
// rearrange other elements
[ 0, 6, 7, 8, 9 ]
> delete nums[0]
true
> nums
[ <1 empty item>, 6, 7, 8, 9 ]
// .find takes a function that identifies an element in an array and returns
// the first instance of the element if it is found. If does not find the
// element, it returns 'undefined'
> let ar = [1, 2, 3, 4, 5]
> let lost = ar.find(e => e === 6)
> lost
> let found = ar.find(e => e === 1)
> found
1
// indexOf() returns the index of the element in the array if the element
// is in the array, or a -1 if it is not present
[ 1, 2, 3, 4, 5 ]
> ar.indexOf(4)
3
> ar.indexOf(9)
-1
// add a second argument to indexOf() that specifies which position to begin
// searching the array
> ar.indexOf(1, 3)
-1
// .lastIndexOf() returns the index of the last occurence of an element in an array
[ 1, 2, 3, 4, 5, 1 ]
> a.lastIndexOf(1)
5
// sort orders arrays alphabetically or numerically
> a = [2, 4, 6, 3, 5, 2, 5, 8]
[
2, 4, 6, 3,
5, 2, 5, 8
]
> a.sort()
[
2, 2, 3, 4,
5, 5, 6, 8
]
[ 'bill', 'albert', 'sara', 'megan' ]
> names.sort()
[ 'albert', 'bill', 'megan', 'sara' ]
// reverse reverses the order of the array elements without sorting
> forward = [0, 1, 2, 3, 4, 5]
[ 0, 1, 2, 3, 4, 5 ]
> forward.reverse()
[ 5, 4, 3, 2, 1, 0 ]
Functions
forEach()
.forEach() executes a function on every item in an array:
const arr = [1, 2, 3]
arr.forEach(e => console.log(e))
// Output:
1
2
3
…spread
The spread operator spreads an array into individual elements. To do this, precede the name of an array with three dots (...):
> let a = [5, 6, 7]
> let nums = [1, 2, 3, 4, ...a]
> nums
[
1, 2, 3, 4,
5, 6, 7
]
You can use the spread operator as function arguments:
> let addTwo = (x, y) => {return x + y}
> let two = [4, 7]
> addTwo(...two)
11
rest parameter
The rest parameter is similar to spread, but it accepts a variable number of arguments:
function someFunction(param1, ...param2) {
console.log(param1, param2);
}
someFunction("hi", "there!", "How are you?");
// Output:
// hi [ 'there!', 'How are you?' ]
Multidimensional arrays
> a1 = [1, 2, 3]
[ 1, 2, 3 ]
> a2 = [4, 5, 6]
[ 4, 5, 6 ]
> a3 = [7, 8, 9]
[ 7, 8, 9 ]
> mAr = [a1, a2, a3]
[ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ] ]
Built-in methods
forEach
arr.forEach(func) // executes func on every element in the array
filter
The filter method takes a function as an argument. This function must return a Boolean. Any item in the array that returns true will be in the filtered array:
let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
let isEven = (el) => {return el % 2 == 0}
let newArr = arr.filter(isEven)
console.log(newArr);
Output:
[ 2, 4, 6, 8, 10 ]
every
The every() method returns a boolean and accepts a function that returns a boolean. It returns true when each element in the array returns a true, and false otherwise:
let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
let isEven = (el) => {return el % 2 == 0}
let everyArr = arr.every(isEven)
console.log(everyArr);
Output:
false
copyWithin
The copyWithin() method can replace a part of an array with another part of an array:
map
map() accepts a function, and executes that function on all elements in an array to return a new array:
let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
let mapArr = arr.map(x => x * 2);
console.log(mapArr);
Output:
[ 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 ]
lastIndexOf
lastIndexOf() returns the index of the last occurence of the argument:
let arr = ["one", "two", "three", "four", "two"]
let lastTwo = arr.lastIndexOf("two")
console.log(lastTwo);
Output:
4
If you pass a value that is not in the array, it returns a -1.