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It depends on how you think about it. I provide a simple method.
When mousemove, get e.target and save it, and then filter out what you want through the function function. It’s that simple

Take Baidu Netdisk as an example. My idea is: because the height of each list item is fixed, you only need to compare the intersection of the rectangle formed when the mouse is dragged with the rectangle of the entire list element, and the amount of calculation will be much smaller. .

The specific method is:

When

1. mousedown, calculate the starting list item index
   2.(offsetTop of 起始的列表項(xiàng) + offsetTop of mouseup) / height of the list item, and then ceil the result to get the selected number.

2. We have the index of the starting list item and the number of selected items, so it is easy to calculate which elements are selected.

The height of the parent container, the scrollTop/list item height of the parent container is fixed, the relative top of the rectangle, and the height of the rectangle
These variables should be enough

按下標(biāo)記關(guān)閉;
按下點(diǎn)坐標(biāo) = {x: null; y: null};
抬起點(diǎn)坐標(biāo) = {x: null; y: null};
已選取元素?cái)?shù)組 = [];
document.onmousedown {
    按下標(biāo)記開啟
    按下點(diǎn)坐標(biāo) = {x: 鼠標(biāo)當(dāng)前橫向偏移; y: 鼠標(biāo)當(dāng)前縱向偏移};
}

待選元素.onmouseover {
    如果按下標(biāo)記開啟，加入已選取元素?cái)?shù)組
}

document.onmouseup {
    按下標(biāo)記關(guān)閉
    清空已選取元素?cái)?shù)組
    抬起點(diǎn)坐標(biāo) = {x: 鼠標(biāo)當(dāng)前橫向偏移; y: 鼠標(biāo)當(dāng)前縱向偏移};
}

利用已選取元素?cái)?shù)組操作已選中的元素
利用按下點(diǎn)坐標(biāo)、抬起點(diǎn)坐標(biāo)繪制框選層

The above is the complete logic, please try to write the code yourself!