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`Array.prototype.splice` changes the contents of an array by removing or replacing existing elements and/or adding new elements in place. It modifies the original array and returns an array containing the removed elements. 

const arr = [1, 2, 3, 4];
const removed = arr.splice(2, 1, 'a', 'b');
console.log(arr); // [1, 2, 'a', 'b', 4]
console.log(removed); // [3]

`Array.prototype.toLocaleString` returns a string representing the array and its elements, formatted according to the locale and options. It uses the `toLocaleString` method of each element. 

const arr = [1, 2, 3];
console.log(arr.toLocaleString()); // '1,2,3' (may vary depending on locale)

`String.prototype.split` splits a string into an array of substrings based on a specified separator. The separator can be a string or regular expression. 

const str = 'a,b,c';
const arr = str.split(',');
console.log(arr); // ['a', 'b', 'c']

`String.prototype.repeat` returns a new string with the specified number of copies of the original string, concatenated together. 

const str = 'abc';
const repeated = str.repeat(3);
console.log(repeated); // 'abcabcabc'

`Array.prototype.forEach` executes a provided function once for each element in the array. It does not return a value and does not modify the original array. 

const arr = [1, 2, 3];
arr.forEach(num => console.log(num));
// Output:
// 1
// 2
// 3

`Array.prototype.concat` merges two or more arrays into a new array. It does not modify the original arrays and can take any number of arguments, including arrays and values. 

const arr1 = [1, 2];
const arr2 = [3, 4];
const merged = arr1.concat(arr2);
console.log(merged); // [1, 2, 3, 4]

`String.prototype.toLowerCase` returns a new string with all characters converted to lowercase. 

const str = 'HELLO';
const lower = str.toLowerCase();
console.log(lower); // 'hello'

`Array.prototype.flat` creates a new array with all sub-array elements concatenated into it recursively up to the specified depth. It helps to flatten nested arrays into a single array. 

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

`Array.prototype.concat` merges two or more arrays into a new array. It does not modify the original arrays and can take any number of arguments, including arrays and values. 

const arr1 = [1, 2];
const arr2 = [3, 4];
const merged = arr1.concat(arr2);
console.log(merged); // [1, 2, 3, 4]

`Array.prototype.pop` removes the last element from an array and returns that element. It modifies the original array. 

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

`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.flatMap` maps each element using a provided mapping function and then flattens the resulting array into a new array. It combines the map and flat operations into a single method. 

const arr = [1, 2, 3];
const flatMapArr = arr.flatMap(x => [x, x * 2]);
console.log(flatMapArr); // [1, 2, 2, 4, 3, 6]

`Array.prototype.reduce` executes a reducer function on each element of the array, accumulating a single result. It takes a callback function and an optional initial value, and returns the final accumulated result. 

const arr = [1, 2, 3];
const sum = arr.reduce((acc, num) => acc + num, 0);
console.log(sum); // 6

`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]

`Array.prototype.flat` creates a new array with all sub-array elements concatenated into it recursively up to a specified depth. It can flatten nested arrays to a specified level. 

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

`String.prototype.bold` returns a string wrapped in HTML `<b>` tags. Note that this method is deprecated and should not be used in modern applications. 

const str = 'hello';
const boldStr = str.bold();
console.log(boldStr); // '<b>hello</b>'

Component lifecycle in functional components is managed using hooks like useEffect, which can perform side effects on mount, update, and unmount. useEffect replaces lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount.

React's Concurrent Mode introduces new rendering capabilities that allow React to interrupt and prioritize rendering work, improving user experience by making updates more responsive. It enables features like Suspense and useTransition, leading to smoother and faster UIs.

Techniques for managing side effects in React include using the useEffect hook for handling async operations, leveraging custom hooks to encapsulate side effect logic, and using libraries like Redux Thunk or Redux Saga for complex side effects management.

useImperativeHandle is a hook used to customize the instance value exposed when using refs. It's useful for controlling what methods or properties are exposed to parent components, such as managing focus or triggering animations from parent components.

useTransition is a hook that allows for deferring updates to a lower priority, improving responsiveness during state transitions. It helps keep the UI responsive by managing updates that can be deferred until more urgent updates are processed.

React Suspense has limitations, including limited support for data fetching and potential performance issues with large components. These can be addressed by using concurrent features like useTransition, combining Suspense with other data fetching libraries, and adopting best practices for component design.

React StrictMode is a development tool that helps identify potential problems in an application by activating additional checks and warnings. It helps catch issues like deprecated APIs, unexpected side effects, and potential problems with components.

Strategies for optimizing React component rendering include using React.memo to prevent unnecessary re-renders, memoizing functions with useCallback, splitting components into smaller pieces, and leveraging virtualized lists for large datasets.

Authentication in React applications is typically handled through tokens or session management with libraries like React Router for protected routes. Authorization involves controlling access to components based on user roles or permissions, often integrated with backend APIs and state management.