How to determine whether an element is within the display area in jquery

When developing web pages, we often need to manipulate DOM elements. To operate on an element, you first need to determine whether the element is within the display area. In a web page, many elements may be hidden and not within the display area. Therefore, before operating these elements, you need to determine whether they are within the display area to avoid operating unnecessary elements and improve page performance.

jQuery provides some methods for determining whether an element is within the display area. Below we will introduce these methods in detail, including judgment methods based on viewports, documents, and elements.

Viewport-based judgment method

In a web page, we can use the viewport (Viewport) to judge whether an element is within the display area. The viewport is the area of ​​the page currently visible to the user, which can be obtained through the properties of the window object.

jQuery provides some methods to determine whether an element is within the viewport, including:

1. $(window).height(): Get the height of the current viewport.
2. $(window).scrollTop(): Gets the distance between the top of the current viewport and the top of the document (that is, the distance the scroll bar scrolls).

Through the above two methods, we can get the top position and bottom position of the current viewport. For an element, if its top position is less than the current viewport bottom position and its bottom position is greater than the current viewport top position, then it is within the display area.

Based on the above principles, we can use code to implement the function of determining whether an element is within the viewport:

function isInViewport(elem) {
    var elemTop = elem.offset().top;
    var elemBottom = elemTop + elem.height();
    var viewportTop = $(window).scrollTop();
    var viewportBottom = viewportTop + $(window).height();
    return elemTop < viewportBottom && elemBottom > viewportTop;
}

// 使用方式
var element = $('#my-element');
if (isInViewport(element)) {
    // 元素在视口内
} else {
    // 元素不在视口内
}

In the above code, the isInViewport() function receives a jQuery element object and obtains the location of the element. The position in the page and the position of the current viewport, determine whether the element is within the display area, and return a Boolean value.

Document-based judgment method

When the page content exceeds the viewport size, the size of the viewport no longer accurately reflects the page content. At this time, it is necessary to judge whether the element is in the display area based on the document. Inside. The document-based judgment method needs to obtain the distance of the element relative to the top of the document and the height of the document to determine whether an element is within the visible area of ​​the document.

jQuery provides the following methods to get the position of the element in the document and the height of the document:

1. $(document).height(): Get the height of the document.
2. elem.offset().top: Gets the distance of the element relative to the top of the document.

Using the above two methods, we can get the top position of the element and the height of the document to determine whether the element is within the visible area of ​​the document. The following is a function that determines whether an element is within the visible area of ​​the document:

function isElementInDocumentViewport(elem) {
    var elemTop = elem.offset().top;
    var elemBottom = elemTop + elem.height();
    var docViewTop = $(window).scrollTop();
    var docViewBottom = docViewTop + $(window).height();
    var docHeight = $(document).height();
    return elemTop < docViewBottom && elemBottom > docViewTop && elemTop > 0 && elemBottom < docHeight;
}

// 使用方式
var element = $(&#39;#my-element&#39;);
if (isElementInDocumentViewport(element)) {
    // 元素在可视区域内
} else {
    // 元素不在可视区域内
}

In the above code, the isElementInDocumentViewport() function determines whether an element is within the visible area of ​​the document, and the function returns a Boolean value. elemTop and elemBottom respectively represent the distance between the top and bottom of the element relative to the top of the document, docViewTop and docViewBottom respectively represent the distance between the top and bottom of the document relative to the top of the viewport, and docHeight represents the height of the document.

Element-based judgment method

Sometimes, when judging whether an element is within the display area, we only care about the situation within a certain parent element. At this time, we can use the element-based judgment method to obtain the height of the parent element and the position of the scroll bar to determine whether the element is within the visible area of ​​the parent element.

The following is a function that determines whether an element is within the visible area of ​​​​the parent element:

function isElementInElementViewport(elem, parentElem) {
    var elemTop = elem.offset().top - parentElem.offset().top;
    var elemBottom = elemTop + elem.height();
    var parentViewTop = parentElem.scrollTop();
    var parentViewBottom = parentViewTop + parentElem.height();
    var parentHeight = parentElem.height();
    return elemTop < parentViewBottom && elemBottom > parentViewTop && elemTop > 0 && elemBottom < parentHeight;
}

// 使用方式
var element = $('#my-element');
var parentElement = $('#my-parent-element');
if (isElementInElementViewport(element, parentElement)) {
    // 元素在父元素可视区域内
} else {
    // 元素不在父元素可视区域内
}

In the above code, the isElementInElementViewport() function receives two jQuery element objects, which need to be determined. element and its parent element. The function returns a Boolean value. elemTop and elemBottom respectively represent the distance between the top and bottom of the element relative to the top of the parent element. parentViewTop and parentViewBottom respectively represent the distance between the top and bottom of the parent element relative to the top of the scroll bar. parentHeight represents the height of the parent element.

Conclusion

In web development, judging whether an element is within the display area is a basic and important function. When using jQuery to operate DOM elements, we can use the methods provided by jQuery to determine whether the element is within the visible area of ​​the viewport, document, or parent element. By using these methods rationally, we can improve page performance and avoid unnecessary operations.

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