Why do events bubble up first before being captured?

The reason is due to the order of event propagation and the event processing mechanism. Event bubbling means that events propagate from the innermost element and propagate outward step by step until they reach the outermost element. The event capture starts from the outermost element and propagates inward step by step until it reaches the innermost element. During the event delivery process, the event first starts from the innermost element of the target element, then passes to the outer elements step by step, and finally reaches the document root node. By bubbling first and then capturing, events can be transmitted and processed on elements at different levels, improving the maintainability of the code.

# Operating system for this tutorial: Windows 10 system, Dell G3 computer.

The reason why events bubble up first and then are captured is due to the order of event propagation and the event processing mechanism.

Event bubbling means that events start to propagate from the innermost element and propagate outward step by step until they propagate to the outermost element. The event capture starts from the outermost element and propagates inward step by step until it reaches the innermost element. In the DOM structure, elements can be nested, for example, one element can contain another element.

The mechanism of event bubbling is to simulate the propagation of events in the real world. When an event occurs on an element, such as a click event, it will start from the innermost element and propagate to the outer elements, just like water ripples spreading. This propagation method makes event processing more flexible, because events can be captured and processed as they bubble up.

Event capture is so that the event can be processed on the outer elements before the event is propagated to the specific element. Sometimes, we may want to perform some processing on outer elements before the event reaches a specific element, such as preventing the event from continuing to propagate or performing some preprocessing operations. The event capture mechanism can meet this need. It can start processing from the outer element before the event reaches the specific element.

Therefore, the mechanism of events bubbling first and then being captured can meet the needs of different scenarios. If we want to do some processing before the event is propagated to specific elements, we can use the event capture mechanism. And if we want to process the event after it is propagated to specific elements, we can use the event bubbling mechanism. In this way, we can choose the appropriate mechanism to handle events based on specific needs.

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