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`String.prototype.concat` combines multiple strings into one string. It does not modify the original strings but returns a new concatenated string. 

const str1 = 'hello';
const str2 = 'world';
const combined = str1.concat(' ', str2);
console.log(combined); // 'hello world'

`String.prototype.match` retrieves the matches of a string against a regular expression. It returns an array of matches or `null` if no matches are found. 

const str = 'hello 123';
const matches = str.match(/\d+/);
console.log(matches); // ['123']

`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.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'

`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'

`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

`String.fromCharCode` returns a string created from the specified sequence of UTF-16 code units. It is used to convert code units to characters. 

const char = String.fromCharCode(65);
console.log(char); // 'A'

`String.prototype.startsWith` checks if a string starts with a specified substring and returns `true` if it does, otherwise `false`. 

const str = 'hello';
console.log(str.startsWith('he')); // true
console.log(str.startsWith('lo')); // false

`String.prototype.endsWith` checks if a string ends with a specified substring and returns `true` if it does, otherwise `false`. 

const str = 'hello';
console.log(str.endsWith('lo')); // true
console.log(str.endsWith('he')); // false

`String.prototype.localeCompare` compares two strings in the current locale and returns a number indicating whether the calling string comes before, after, or is equal to the compared string. 

const str1 = 'apple';
const str2 = 'banana';
const result = str1.localeCompare(str2);
console.log(result); // -1 (str1 is less than str2)

`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>'

`String.prototype.link` creates an HTML `<a>` element wrapping the string, which is used to create hyperlinks. This method is deprecated and should not be used in modern applications. 

const str = 'Click here';
const linkedStr = str.link('https://example.com');
console.log(linkedStr); // '<a href="https://example.com">Click here</a>'

`String.prototype.fontcolor` returns a string wrapped in HTML `<font>` tags with a specified color. This method is deprecated and should not be used in modern applications. 

const str = 'hello';
const coloredStr = str.fontcolor('red');
console.log(coloredStr); // '<font color="red">hello</font>'

`String.prototype.fontsize` returns a string wrapped in HTML `<font>` tags with a specified size. This method is deprecated and should not be used in modern applications. 

const str = 'hello';
const sizedStr = str.fontsize(7);
console.log(sizedStr); // '<font size="7">hello</font>'

`String.prototype.anchor` creates an HTML `<a>` element wrapping the string with a specified name attribute. This method is deprecated and should not be used in modern applications. 

const str = 'Click here';
const anchoredStr = str.anchor('top');
console.log(anchoredStr); // '<a name="top">Click here</a>'

`String.prototype.small` returns a string wrapped in HTML `<small>` tags. This method is deprecated and should not be used in modern applications. 

const str = 'hello';
const smallStr = str.small();
console.log(smallStr); // '<small>hello</small>'

Common pitfalls with useEffect include missing dependency arrays, leading to infinite loops or stale closures. Ensuring correct dependencies and cleaning up side effects properly with cleanup functions can help avoid these issues and ensure correct behavior.

React DevTools is a browser extension that provides insights into React component hierarchies, state, and props. It aids in debugging by allowing developers to inspect component trees, view hooks and context, and profile performance to identify and resolve issues.

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 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.

Optimizing React's context API involves strategies like using separate contexts for different state slices, memoizing context values, and avoiding deep nesting of contexts. Additionally, consider using useReducer for managing complex context state to minimize unnecessary re-renders.

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.

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.

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.

useContext can simplify global state management by allowing components to access context values directly. However, it can lead to performance issues due to re-renders of all consumers when context values change. For complex state, consider using dedicated state management libraries.