Flash cards
Review the key moves
What is the main idea behind ECMAScript 2024?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
// ___ an ArrayPut the learning moves in the order that makes the concept easiest to apply.
New Features in JavaScript 2024
| Feature | Description |
|---|---|
| Object.groupBy() | Groups object elements according to values returned from a callback function |
| Map.groupBy() | Groups map elements according to values returned from a callback function |
| String isWellFormed() | Returns true if a string is well formed |
| String.toWellFormed() | Returns a new string where "lone surrogates" are replaced with Unicode U+FFFD |
| Promise.withResolvers() | |
| Atomics waitAsync |
Warning
These features are relatively new.
Older browsers may need an alternative code (Polyfill)
JavaScript Object.groupBy()
Example
// Create an Array
const fruits = [ {name:"apples", quantity:300}, {name:"bananas", quantity:500}, {name:"oranges", quantity:200}, {name:"kiwi", quantity:150}
];
// Callback function to Group Elements function myCallback({ quantity }) {
return quantity > 200 ? "ok" : "low";
}
// Group by Quantity
const result = Object.groupBy(fruits, myCallback);The Object.groupBy() method groups elements of an object according to string values returned from a callback function.
The Object.groupBy() method return a new object.
The Object.groupBy() method does not change the original object.
The elements in the original and in the returned object are the same.
Future changes will be reflected in both the original and in the returned object.
JavaScript Map.groupBy()
Example
// Create a Map
const fruits = [ {name:"apples", quantity:300}, {name:"bananas", quantity:500}, {name:"oranges", quantity:200}, {name:"kiwi", quantity:150}
];
// Callback function to Group Elements function myCallback({ quantity }) {
return quantity > 200 ? "ok" : "low";
}
// Group by Quantity
const result = Map.groupBy(fruits, myCallback);The Map.groupBy() method groups elements of a map according to string values returned from a callback function.
The Map.groupBy() method returns a new map.
The Map.groupBy() method does not change the original object.
The elements in the original and in the returned object are the same.
Future changes will be reflected in both the original and in the returned object.
Object.groupBy() vs Map.groupBy()
The difference between Object.groupBy() and Map.groupBy() is:
Object.groupBy() groups elements into a JavaScript object.
Map.groupBy() groups elements into a Map object.
JavaScript String isWellFormed()
The isWellFormed() method returns true if a string is well formed.
Otherwise it returns false .
A string is not well formed if it contains lone surrogates .
Examples
let text = "Hello world!";
let result = text.isWellFormed();Lone Surrogates
A lone surrogate is a Unicode surrogate code point that is not part of a valid surrogate pair used to represent characters in UTF-16 encoding.
JavaScript String toWellFormed()
Examples
let text = "Hello World \uD800";
let result = text.toWellFormed();JavaScript Promise.withResolvers()
Promise.withResolvers() is a static method that simplifies the creation and management of Promises.
Promise.withResolvers() provides a more convenient way to access the resolve and reject functions associated with a Promise outside of its executor function.
Instead of the traditional new Promise((resolve, reject) => { ... }) constructor pattern, Promise.withResolvers() returns an object containing:
- promise: The newly created Promise instance
- resolve: A function to fulfill the promise with a value
- reject: A function to reject the promise with a reason (error)
Example
<p id="demo">Waiting...</p> <script> const {promise, resolve, reject} = Promise.withResolvers();
// You can now use 'resolve' and 'reject' anywhere // in your code to control the state of 'promise'. // Simulate async work setTimeout(() => {
const success = Math.random() > 0.5;
if (success) {
resolve("Operation successful!");
} else {
reject("Operation failed!");
}
}, 1000);
// Update the UI when the promise finishes promise .then((message) => { document.getElementById("demo").innerHTML = message;
}) .catch((error) => {
document.getElementById("demo").innerHTML = error; ;
});
</script>Example Explained
- The <p id="demo"> initially shows "Waiting..."
- After 1 second, the promise resolves or rejects
- The result is written into "demo"