ES2022新特性一览

• .at() method on built-in indexables
• RegExp Match Indices
• Object.hasOwn
• Error cause
• Top-level await
• Class field declarations
• Ergonomic brand checks for private fields

.at() method on built-in indexables

.at method on built-in indexables 📕.

const cart = ['🍎', '🍌', '🍍'];

// first element
cart.at(0); // '🍎'

// last element
cart.at(-1); // '🍍'

// out of bounds
cart.at(-100); // undefined 
cart.at(100); // undefined 

const int8 = new Int8Array([0, 10, 42, -10]);

// first element 
int8.at(0); // 0

// last element
int8.at(-1); // -10

// out of bounds
int8.at(-100) // undefined 
int8.at(100) // undefined

const sentence = 'This is a sample sentence'

// first element 
sentence.at(0); // 'T'

// last element
sentence.at(-1); // 'e'

// out of bounds
sentence.at(-100) // undefined
sentence.at(100) // undefined

RegExp Match Indices

additional information about the start and end indices of captured substrings 📕

/(?<xs>x+)(?<ys>y+)/.exec('xxxyyxx');
/*[
  'xxxyy',
  'xxx',
  'yy',
  index: 0,
  input: 'xxxyyxx',
  groups: [Object: null prototype] { xs: 'xxx', ys: 'yy' }
]*/

let input = "abcd";
let match = /b(c)/.exec(input);
let indices = match.indices;

// `indices` has the same length as match
indices.length === match.length

// The first element of `indices` contains the start/end indices of the match
indices[0]; // [1, 3];
input.slice(indices[0][0], indices[0][1]); // same as match[0]

// The second element of `indices` contains the start/end indices of the first capture
indices[1]; // [2, 3];
input.slice(indices[1][0], indices[1][1]); // same as match[1]);

Object.hasOwn

Object.hasOwn 📕

let books = {}
books.prop = 'exists';

// `hasOwn` will only return true for direct properties:
Object.hasOwn(books, 'prop');             // returns true
Object.hasOwn(books, 'toString');         // returns false
Object.hasOwn(books, 'hasOwnProperty');   // returns false

// The `in` operator will return true for direct or inherited properties:
'prop' in books;                          // returns true
'toString' in books;                      // returns true
'hasOwnProperty' in books;                // returns true

Error cause

cause property indicating the cause of an error. 📕

const actual = new Error('a better error!', { cause: 'Error cause' });

actual instanceof Error; // true
actual.cause; // 'Error cause'

try {
  maybeWorks();
} catch (err) {
  throw new Error('maybeWorks failed!', { cause: err });
}

Top-Level await

await outside of async functions in modules 📕

// say this is index.mjs

// fails
await Promise.resolve('🍎');
// → SyntaxError: await is only valid in async function

// fix with wrapping
(async function() {
  await Promise.resolve('🍎');
  // → 🎉
}());

// to top-level await
await Promise.resolve('🍎') // '🍎'

const i18n = await import(`./content-${language}.mjs`);

Class field declarations

Orthogonally-informed combination of public and private fields. 📕

class SampleClass {
    /*
      instead of:
      constructor() { this.publicID = 42; }
    */
    publicID = 42; // public field

    /*
      instead of:
      static get staticPublicField() { return -1 }
    */
    static staticPublicField = -1;

    // static private field
    static #staticPrivateField = 'private';

    //private methods
    #privateMethod() {}

    // static block
    static {
      // executed when the class is created
    }
}

ergonomic brand checks for private fields

Brand checks without exceptions. 📕

class C {
  #brand;

  #method() {}

  get #getter() {}

  static isC(obj) {
    // in keyword to check
    return #brand in obj && #method in obj && #getter in obj;
  }
}

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