Use .size() or .length() in JavaScript?

Let's take a closer look....size() is actually just calling .length (the jQuery source code shown below clearly shows this), so we save a function call:

//https://code.jquery.com/jquery-latest.js
// 匹配元素集中包含的元素?cái)?shù)量
size: function() {
    return this.length;
},

Tests proved that .length() is faster than .size()(jsperf test resultshttp://www.miracleart.cn/link/5208093bcaf65dfea07bdab31d600223 Why are there .size() functions in jQuery What? My initial guess is that they keep it abstract so that it can provide a backward compatible API if you need a different .length calculation method in the future. You may have seen common usages:

// 檢查 DOM 元素是否存在
if ($('#id').length > 0) { ... }

In short, I will use it all the time unless someone gives me a good reason not to use .length.

FAQ for JavaScript Size and Length (FAQ)

What is the difference between size and length in JavaScript?

In JavaScript, both size and length are used to determine the number of elements in an object. However, they are used for different types of objects. The length attribute is used for array objects, while the size attribute is used for Map and Set objects. The length property of the array returns the number of elements in the array. On the other hand, the Map or Set object's size properties return the number of key-value pairs or elements in the object, respectively.

How to determine the size of an object in JavaScript?

Unlike arrays, objects in JavaScript do not have built-in size or length properties. However, you can use the Object.keys() method to determine the size of the object (i.e. the number of attributes it has). This method returns an array of the object's own attribute names. You can then get the length of this array to find out the size of the object.

Can I use the length attribute with strings in JavaScript?

Yes, you can use the length attribute with strings in JavaScript. The length property of a string returns the number of characters in the string, including spaces and special characters.

Why is the length of the JavaScript hollow array 0?

In JavaScript, the length attribute of the array reflects the number of elements in the array. An empty array contains no elements, so its length is 0.

Can I change the length of an array in JavaScript?

Yes, you can change the length of an array in JavaScript. If you increase the length, a new element with a value of undefined will be added to the end of the array. If you reduce the length, the element is removed from the end of the array.

What happens if I try to access an array element with an index greater than or equal to the length of the array?

If you try to access an array element with an index greater than or equal to the length of the array, JavaScript will return undefined. This is because the index is beyond the range of the array.

Can I use the size attribute with arrays in JavaScript?

No, you cannot use the size attribute with arrays in JavaScript. The size attribute is used for Map and Set objects, not arrays. To determine the number of elements in an array, you should use the length attribute.

How to determine the size of Map or Set objects in JavaScript?

You can use the size attribute to determine the size of the Map or Set object in JavaScript. This property returns the number of key-value pairs in the Map object or the number of elements in the Set object.

Can I use the length attribute with Map or Set objects in JavaScript?

No, you cannot use the length attribute with Map or Set objects in JavaScript. The length attribute is used for arrays and strings, not Map or Set objects. To determine the number of key-value pairs in the Map object or the number of elements in the Set object, you should use the size attribute.

What is the difference between size attributes and length attributes in terms of performance in JavaScript?

There is no significant difference between the size attribute and the length attribute in JavaScript in terms of performance. Both properties provide a quick and efficient way to determine the number of elements in an object. However, the choice between the two depends on the type of object you are using. The length attribute is used for arrays and strings, while the size attribute is used for Map and Set objects.

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