This article will help you understand the use of pseudo-arrays in JavaScript

Preface

When we learn JavaScript, we often encounter a concept: Pseudo Array. The so-called pseudo-array refers to objects with an array-like structure, but not a real array. In this article, we will introduce the characteristics and features of pseudo-arrays in detail and provide some JavaScript code examples.

1. Characteristics and features of pseudo-arrays

Subscript index: A pseudo-array can access its elements through a numeric index, just like an array. Normally, the index of a pseudo-array is incremented from 0. However, unlike arrays, the index of a pseudo-array does not have the methods and properties of the array.

Length attribute: Pseudo-array objects usually have an attribute that represents length, for examplelength. This property will be automatically updated based on the number of elements in the pseudo-array. It should be noted that although the pseudo-array has a length property, it cannot pass through like an array.push()orpop()Change the length by other methods.

Type judgment: The type of pseudo-array is usuallyObject, notArray. This is because pseudo-arrays are not real arrays, they just simulate the structure and behavior of the array.

Method limitation: Pseudo-array objects do not have many methods for arrays. For example, it does notforEach()、map()、filter()Contour higher order functions. At the same time, it doesn'tpush()、pop()、splice()etc. for modifying the content of the array.

2. The role of pseudo-arrays

Although pseudo-arrays are not real arrays, they are still very useful in some specific scenarios. Here are some common situations and uses for using pseudo-arrays:

• argumentsObject: Inside a function, a pseudo-array can be usedargumentsto access the parameters of the passed function. AlthoughargumentsNot a real array, but it allows access to parameters through indexes and haslengthAttributes to represent the number of parameters. This allows us to write functions that can accept any number of parameters.
• DOM element collection: When we use JavaScript to manipulate elements on a web page, for example()or()The obtained element collection, they return a pseudo-array. We can manipulate elements in iterating over or indexing access to the pseudo-array.
• String: A string is treated as a pseudo-array similar to a character array in JavaScript. We can access a single character in a string through an index and uselengthAttribute gets the length of the string. This allows us to conveniently perform string operations, such as intercepting substrings, traversing characters, etc.
• Class array object: Sometimes, we might customize an object to simulate the behavior of an array, such as a dictionary or hash table. These custom class array objects have an array-like structure that can access elements through indexes, while also havinglengthproperty. Although these objects are not real arrays, they can replace the functionality of arrays in a specific scenario.

In general, pseudo-arrays provide array-like behavior and structure in certain specific cases, allowing us to access elements through indexes and have length attributes. Although not real arrays, pseudo-arrays play an important role in function parameters, DOM operations, string processing, and custom objects.

3. Pseudo-array code examples

// Example 1: arguments objectfunction sum() {
  var total = 0;
  for (var i = 0; i < ; i++) {
    total += arguments[i];
  }
  return total;
}
(sum(1, 2, 3, 4)); // Output: 10
// Example 2: DOM element collectionvar divs = ('div');
(); // Output: Number of elements(divs[0]); // Output: The first div element
// Example 3: Stringvar str = 'Hello, World!';
(); // Output: string length(str[0]); // Output: First character
// Example 4: Class array objectvar obj = { 0: 'a', 1: 'b', 2: 'c', length: 3 };
for (var i = 0; i < ; i++) {
  (obj[i]);
}

The above code shows different types of pseudo-arrays:argumentsObjects, DOM element collections, strings, and custom class array objects. They all have array-like features and features, but are not real arrays.

Summarize

A pseudo-array is an object with an array structure, but does not have all the methods and properties of an array. They can access elements through numeric indexes and usually have an attribute representing length. Although pseudo-arrays are very useful in some scenarios, we need to pay attention to the difference between them and real arrays to avoid incorrect use.

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