index questions

`Array.prototype.fill` fills all elements of an array from a start index to an end index with a static value. It modifies the original array and returns the updated array. 

const arr = [1, 2, 3, 4];
arr.fill(0, 1, 3);
console.log(arr); // [1, 0, 0, 4]

`Array.prototype.fill` fills all the elements of an array from a specified start index to an end index with a static value. It modifies the original array and returns the updated array. 

const arr = [1, 2, 3, 4];
arr.fill(0, 1, 3);
console.log(arr); // [1, 0, 0, 4]

`Array.prototype.lastIndexOf` returns the index of the last occurrence of a specified element within the array. If the element is not found, it returns `-1`. It performs a strict comparison (===). 

const arr = [1, 2, 3, 2];
console.log(arr.lastIndexOf(2)); // 3
console.log(arr.lastIndexOf(4)); // -1

`Array.prototype.indexOf` returns the index of the first occurrence of a specified element within the array. If the element is not found, it returns `-1`. It performs a strict comparison (===). 

const arr = ['a', 'b', 'c'];
console.log(arr.indexOf('b')); // 1
console.log(arr.indexOf('d')); // -1

`String.prototype.indexOf` returns the index of the first occurrence of a specified value in a string. If the value is not found, it returns -1. 

const str = 'hello';
const index = str.indexOf('l');
console.log(index); // 2

`String.prototype.substr` extracts a substring from a string based on a starting index and length. It returns the extracted substring. 

const str = 'hello world';
const substr = str.substr(6, 5);
console.log(substr); // 'world'

`Array.prototype.entries` returns a new Array Iterator object that contains the key/value pairs for each index in the array. It allows iteration over the array's indices and values. 

const arr = ['a', 'b', 'c'];
const iterator = arr.entries();
for (const [index, element] of iterator) {
  console.log(index, element);
}
// Output:
// 0 'a'
// 1 'b'
// 2 'c'

`Array.prototype.values` returns a new Array Iterator object that contains the values for each index in the array. It allows iteration over the array's values. 

const arr = ['a', 'b', 'c'];
const iterator = arr.values();
for (const value of iterator) {
  console.log(value);
}
// Output:
// 'a'
// 'b'
// 'c'

`String.prototype.charAt` returns the character at a specified index in a string. If the index is out of range, it returns an empty string. 

const str = 'hello';
const char = str.charAt(1);
console.log(char); // 'e'

`String.prototype.search` searches a string for a match against a regular expression and returns the index of the first match. If no match is found, it returns -1. 

const str = 'hello 123'; 
const index = str.search(/\d+/); 
console.log(index); // 6 

`String.prototype.codePointAt` returns an integer representing the UTF-16 code unit at a specified index in a string. It is useful for dealing with Unicode characters. 

const str = '𠮷';
const codePoint = str.codePointAt(0);
console.log(codePoint); // 134071

`Array.prototype.findIndex` returns the index of the first element in the array that satisfies the provided testing function. If no elements satisfy the function, it returns `-1`. It does not modify the original array. 

const arr = [5, 12, 8];
const index = arr.findIndex(num => num > 10);
console.log(index); // 1

`Array.prototype.keys` returns a new Array Iterator object that contains the keys (indices) for each index in the array. It allows iteration over the array's indices. 

const arr = ['a', 'b', 'c'];
const iterator = arr.keys();
for (const key of iterator) {
  console.log(key);
}
// Output:
// 0
// 1
// 2

`Array.prototype.fill` changes all elements in an array to a static value from a start index to an end index. It modifies the original array. 

const arr = [1, 2, 3];
arr.fill(0, 1, 3);
console.log(arr); // [1, 0, 0]

To create an index on multiple columns, use the `CREATE INDEX` statement and specify the columns separated by commas. For example, to create an index on the 'last_name' and 'first_name' columns of the 'employees' table, you would use `CREATE INDEX idx_name ON employees(last_name, first_name);`. Multi-column indexes can speed up queries that filter on these columns together.

Wildcard indexes in MongoDB enable indexing of fields within documents that may have unpredictable structures. They allow querying on any field without explicitly defining all possible fields. For instance, `db.collection.createIndex({'$**': 1})` creates a wildcard index, which is useful for collections with varying schema attributes.

A database index is a data structure that improves the speed of data retrieval operations. It works by creating a sorted representation of the indexed column(s). For instance, indexing the 'email' column allows for quick lookups of user accounts based on their email addresses.