05 JAVASCRIPT ARRAYSUI 2023-2024

Arrays in JavaScript

The Array object is a class that enables you to store a set of elements in order under a single variable name. This class has methods to perform the most common Array operations.

Array in JS have a variable length and can store elements of any type. You can access an arbitrary position of the array, without being consecutive with the last item. This fact implies you can have a non-dense array.

let myList = [0, 1, 2];
myList[6] = 6;
console.log(myList.length);
console.log(myList);
for (let pos of myList) {
    console.log(pos);
}

Also, we can make the array smaller by changing the array length

let myList = [];
let n = 100;
console.log(myList.length);
for (let i = 0; i < n; ++i) myList.push(i); // append the new value
console.log(myList.length);
myList.length = 3;
console.log(myList);
console.log(myList.length);

Iterate over an array

You can iterate over all the elements from the array in different ways, by using the JavaScript for syntax

let myList = [1, 2, 3];

for (let i = 0; i < myList.length; ++i)
    console.log(myList[i]) // access an item by its position

for (let value of myList)
    console.log(value) // value is assigned in each iteration to the next item in the array

for (let pos in myList)
    console.log(pos) // Iter over the keys of the array, in this case, the position

Some methods from the Array class

concat: Concatenates 2 arrays and returns a new one

let a = [1, 2, 3],
    b = [4, 5, 6];
let c = a.concat(b);
console.log(c);
console.log(a);

every: This method gets a function as an argument and returns true if all the items in the array return a non-false value.

let a = [2, 6, "hola", []];
function test(item) {
    console.log(item);
    return item;
}
console.log(a.every(test));
a.push("");
console.log(a.every(test));

filter: Returns a new array with the items that return true by the provided function.

let old = [];
 for (let i = 0; i < 50; ++i) old.push(i);
 let noPrime = new Set();
 let prime = old.filter(function(number) {
     if (number in noPrime) return false;
     for (let i = 2, s = Math.sqrt(number); i <= s; ++i) {
         if (number % i === 0) {
             noPrime.add(i);
             noPrime.add(number/i);
             return false;
         }
   }
   return true
 });
console.log(prime);

fill: Changes items of the array with a static value into the range passed as arguments

let a = [];
a.length = 10;
console.log(a);
let value = 0,
    startPos = 4,
    endPos = 6;
console.log(a.fill(value, startPos, endPos));

find, findLast, findIndex: These methods get a callback function and return the first and last item, respectively, from the array which the callback function return true. In the case of findIndex, it returns the position of the first matching item.

let a = [1, 2, 3, 5, 7, 10, 11];
console.log(a.find(function(item) {
    return item%5 === 0
}));

console.log(a.findLast(function(item) {return item%5 === 0}));

console.log(a.findIndex(function(item) {
    return item%5 === 0
}));

forEach: Executes a function for each element in the array, without editing it
includes: check if an item is in the array
indexOf, lastIndexOf: return the position of the first or last item found in the array; -1 if the argument does not exist.
join: This method joins all items from the array using the character passed as argument

map: Run a function for each item of the array and return a new one with the same length and the values returned by the function

let names = "john smith";
let tmpNames = names.split(" ").map(function(name) {
    return name[0].toUpperCase() + name.slice(1);
});
console.log(tmpNames.join(" "));

pop: Returns the last item from the array and remove it

push: append a new item (or a list of items)

let a = [];
a.push(1,2,3);
console.log(a);

reduce, reduceRight: Return a value as the result of an operation repeated across all elements from the array

let first = [];
for (let i = 0; i < 100; ++i)
    first.push(i + 1);
function sumOfNFirst(n) {
    return n*(n+1)/2;
}

let sum = first.reduce(function(accumulator, actual) {
    return accumulator + actual;
}, 0);
console.log(sum === sumOfNFirst(100));

reverse: Return a new array with the order reverted
shift : removes the first or last element from the array and returns it
slice: This function gets 2 arguments and returns a new array from the first to the last argument. If the last argument is omitted, get all the items from the value passed.
```
let first = [];
  for (let i = 0; i < 100; ++i)
      first.push(i + 1);
console.log(first.slice(1,5));
```
some: Return true if any item pass a condition
sort: Sort the array in place, in order ascending and with a conversion to UTF-16 char. If you want to change the sort function, you can pass as an argument a function that gets as arguments 2 consecutive items in the array. If the function returns a negative value, it implies that the first argument goes before the second argument. Otherwise, the second argument goes before.
```
let a = [1, 5, 4, 10, 2, 60, 7];
let copy = [1, 5, 4, 10, 2, 60, 7];
let b = a.sort();
//console.log(a)
//console.log(a === b);
copy.sort(function(a, b) {
    return a - b
});
console.log(copy);
```

splice: Edit the content of an array by removing, editing, or adding elements to the original array. This function returns a new array with the removed items.

let a = [1, 5, 4, 10, 2, 60, 7];
let b = [1, 5, 4, 10, 2, 60, 7];
let c = [1, 5, 4, 10, 2, 60, 7];
let start = 0,
    editCount = 0,
    replaceValue = null;
// remove all elements from the start position
console.log(a.splice());
console.log(a);
// Insert a new item in an arbitrary position
console.log(b.splice());
console.log(b);
// Remove an item and insert other one
console.log(c.splice())
console.log(c);

unshift: insert a new item as the first position of the array

References:

JavaScript: The definitive guide : Master the world’s most-used programming language
MDN Arrays