Array.from() is a method used in JavaScript to convert class arrays or iterable objects into real arrays. Its core uses include: 1. Convert class array objects (such as arguments or NodeList) into arrays for easy subsequent operations; 2. Split strings into character arrays, especially to correctly handle Unicode characters; 3. Quickly generate arrays of specified lengths, and can fill in the initial value with map functions; 4. Data transformation is performed through mapping functions during the conversion process, supporting direct conversion and mapping from Set, Map and other structures, improving code simplicity and functionality.
Array.from() is a very practical method in JavaScript to convert array-like objects or iterable objects into real arrays. It is not only simple, but also solves many practical problems, such as handling arguments objects, DOM node lists, etc.
Let’s take a look at several common and practical usage scenarios and usages.
1. Convert class array object to array
Class array objects refer to objects that have length attributes but are not real arrays, such as arguments in functions or NodeList in DOM.
function example() {
console.log(Array.isArray(arguments)); // false
const args = Array.from(arguments);
console.log(Array.isArray(args)); // true
}
example(1, 2, 3);This conversion is very useful when dealing with class array structures returned by old browsers or some APIs.
2. Split the string into a character array
A string itself is not an array, but it is iterable. Using Array.from() can easily split strings into character arrays, including processing Unicode characters, and can also be correctly recognized.
Array.from('hello'); // ['h', 'e', 'l', 'l', 'o']
In contrast, split('') can also achieve similar effects, but Array.from() is more reliable when dealing with certain special characters (such as emojis).
3. Quickly generate an array of specified lengths
If you just want to generate an empty array of specified length (or fill in the default value), you can use it in conjunction with the { length } object.
Array.from({ length: 5 }); // [undefined, undefined, undefined, undefined, undefined] You can also use the map function to fill in the initial value:
Array.from({ length: 5 }, (_, i) => i 1); // [1, 2, 3, 4, 5]This technique is often used to generate indexes, ID lists, form fields, etc.
4. Combining map function for mapping conversion
The second parameter of Array.from() is a mapping function, which can be understood as performing a map operation while generating an array.
const numbers = Array.from([1, 2, 3], x => x * 2); // [2, 4, 6]
You can also convert directly from other structures like this:
- Create an array from Set and map:
const set = new Set([1, 2, 3]); Array.from(set, x => x * 2); // [2, 4, 6]
- Extract an array of key-value pairs from Map:
const map = new Map([['a', 1], ['b', 2]]); Array.from(map); // [['a', 1], ['b', 2]]
Basically these common uses. Array.from() looks simple, but is flexible to use, especially when dealing with non-standard array structures. After mastering it, you will find that many structures that originally needed to be processed manually can be solved with one line of code.
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