String.prototype.trimStart() and trimEnd()

ECMAScript 10 introduced the `trimStart` and `trimEnd` methods for precise control over whitespace trimming in strings. These methods serve as aliases for `trimLeft` and `trimRight`, offering a more intuitive naming convention. Together with the `trim` method, they form a complete trio, allowing developers to selectively handle whitespace at the beginning or end of strings. `trimStart` removes leading whitespace, while `trimEnd` handles trailing whitespace. The whitespace characters they remove include spaces, tabs, line breaks, and all other whitespace characters. Practical use cases include form input processing, log file handling, and multiline template string processing. In terms of performance, these methods are more efficient than regular expressions. For browser compatibility, modern browsers fully support them, while polyfills can be used in older environments. These methods are particularly well-suited for functional programming and chainable calls, and they correctly handle various Unicode whitespace characters. They also prove valuable in template tag functions, data cleaning, internationalization processing, and other scenarios.

Object.fromEntries()

`Object.fromEntries` is a static method introduced in ECMAScript 10, used to convert a list of key-value pairs into an object. It serves as the inverse operation of `Object.entries` and can be combined with it to achieve shallow object copying or property filtering. This method can directly handle iterable objects like `Map` structures and `URLSearchParams`. In practical applications, it is commonly used for parsing URL query parameters, processing form data, and converting data formats. Note that keys are coerced into strings. Since ES2019, it supports objects with `Symbol` keys and does not inherit additional prototype properties. Compared to `reduce` implementations, it offers a more concise approach. In unsupported environments, compatibility can be achieved through polyfills.

Array.prototype.flat() and flatMap()

The addition of the `flat` and `flatMap` methods in ECMAScript 10 greatly simplifies nested array processing. The `flat` method can flatten multi-dimensional arrays into one-dimensional arrays, supports specifying the flattening depth, and automatically removes empty slots. The `flatMap` method combines mapping and shallow flattening, making it particularly suitable for scenarios requiring both mapping and expansion. Compared to the traditional combination of `map` followed by `flat`, it offers better performance by avoiding the creation of intermediate arrays. These methods are widely used in e-commerce data processing, flattening tree structures, and functional programming. While modern browsers generally support them, polyfills may be needed for older environments. When handling large datasets, performance optimization should be considered. Many programming languages have similar concepts to `flatMap`, though they may go by different names.

Line separator and paragraph separator

ECMAScript 9 introduced explicit support for the line separator U+2028 and paragraph separator U+2029, standardizing line break handling in strings and regular expressions to address cross-platform inconsistencies in line terminators. The Unicode standard defines these special characters as representing logical line endings and paragraph endings, respectively. Before ES9, these separators could cause syntax errors or JSON parsing issues. ES9 explicitly treats them as valid characters, improving string literals, regular expressions, and JSON specifications. Practical applications include multiline text processing and source code validation tools. Regular expressions also enhanced dot matching and whitespace detection. Major JavaScript engines have fully implemented this feature, though performance tests show heavy usage may impact operation speed. TypeScript has supported these features since version 3.2, requiring corresponding compiler options, and its type system can correctly identify these special characters.

JSON superset supports this statement

ECMAScript 9 introduced significant enhancements to JSON parsing, enabling full support for the JSON superset, particularly addressing the previous inability to handle unescaped line separators (U+2028) and paragraph separators (U+2029). This improvement allows `JSON.parse()` to process a broader range of valid JSON text while maintaining compatibility with existing specifications. In practical applications, developers can more conveniently handle multiline strings and template literals containing these special separators. This feature works in tandem with other ES9 additions like regex improvements and asynchronous iteration. Modern browsers and Node.js now fully support it, and TypeScript has provided complete support since version 3.2. Despite the enhanced functionality, security boundaries must still be observed—such as using `try-catch` and sanitizing untrusted data. Originating from a TC39 proposal, this feature underwent the full standardization process. Development tools like Babel and ESLint have also been adapted to support it. Compared to the JSON5 specification, ES9's JSON superset extends support for special characters while preserving strictness, offering a better solution for scenarios like internationalized message processing.

Template strings restrict escape sequences

ECMAScript 9 imposed stricter restrictions on escape sequences in template strings primarily for security and code consistency considerations. The new specification clarifies legal versus illegal escape sequences. Earlier versions allowed certain illegal escape sequences, which could lead to unexpected behavior or security vulnerabilities. Legal escape sequences include line breaks, tabs, backslashes, backticks, interpolation expression literals, Unicode, and hexadecimal escape sequences. Illegal escape sequences, such as undefined 'a' or 'z', will throw syntax errors. In strict mode, the restrictions are even tighter, prohibiting octal escape sequences. When dynamically generating template strings, avoid illegal escape sequences by using Unicode code points or string concatenation as alternatives. Tag functions still apply these restrictions during processing. Compared to other languages, ECMAScript 9's restrictions are more stringent.

The dotAll mode in regular expressions translates this sentence into English.

ECMAScript 9 introduced the dotAll mode for regular expressions via the `s` flag, enabling the dot metacharacter to match all characters, including line terminators, addressing the limitation where traditional regex dots could not match newlines. Developers previously had to use workarounds like `[\s\S]` to bypass this constraint. The dotAll mode simplifies multiline text processing, particularly improving efficiency when handling template literals or HTML tag content. This mode can be combined with other flags like `m` and `u`, though potential performance impacts should be noted. Modern JavaScript environments, including Node.js and major browsers, now support this feature. The `flags` property can be used to check whether a regular expression has the dotAll mode enabled.

Regular expression lookbehind assertion

ECMAScript 9 introduced reverse assertions in regular expressions, including positive lookbehind assertions and negative lookbehind assertions, which are used to match strings that are preceded or not preceded by a specific pattern. Positive lookbehind assertions use the syntax `(?<=...)` to match positions where the preceding text must conform to the pattern, while negative lookbehind assertions use the syntax `(?<!...)` to match positions where the preceding text must not conform to the pattern. Reverse assertions can be used to match numbers following a currency symbol or content after specific words, as well as numbers not preceded by a currency symbol or words not inside quotation marks. They can also be combined with capturing groups to achieve complex matching logic, though this may impact performance, especially when processing long strings. Modern browsers support reverse assertions, but older environments may require transpilation. Reverse assertions are highly useful in form validation, log analysis, and text processing—for example, validating that a password does not contain a username or extracting error messages following a specific timestamp.

Regular expression named capture groups

ECMAScript 9 introduced named capture groups in regular expressions, enhancing code readability and maintainability through named identifiers. Developers can define named capture groups using the question mark and angle bracket syntax and access matching results via the `groups` property. This feature supports destructuring assignment to simplify field extraction. In string replacement, named groups can be referenced using a dollar sign followed by angle brackets. Within the regex, backreferences are implemented using a backslash followed by `k` and angle brackets. Named capture groups can be combined with newer features like Unicode property escapes. While their performance is comparable to traditional capture groups, they significantly improve the readability of complex patterns. Practical applications include parsing structured text, such as log analysis and phone number processing. Usage considerations include ensuring unique group names and addressing compatibility with older environments. Best practices involve using meaningful group names, providing default values, and considering transpilation solutions. This feature greatly improves the development experience with regular expressions.

Rest/Spread properties

ECMAScript 9 (ES2018) introduced enhancements to Rest/Spread properties, simplifying object and array operations. The Rest parameter in object destructuring allows collecting remaining enumerable properties, which is particularly useful for handling objects with an uncertain number of properties. Spread properties enable expanding the enumerable properties of an object into another object, achieving shallow copying and merging of objects. The Rest/Spread operations for arrays were also handled more consistently in ES9. Practical applications include function parameter handling, object merging and overriding, and passing React component props. It’s important to note considerations such as prototype chain properties, non-enumerable properties, performance implications, and differences from `Object.assign`. Rest/Spread properties can also be combined with other ES9 features like asynchronous iteration and `Promise.finally`. TypeScript provides full type support for Rest/Spread properties.