JS execution mechanism

js After code execution , In the main thread execution stack, tasks are divided into two categories ;
One is Synchronization task , The other is Asynchronous task ; The main thread execution stack gives priority to synchronization tasks , Asynchronous tasks are put into pending handlers , When the conditions are met , Be put in Message queue / Task queue in , After the main thread executes all synchronization tasks in the stack , adopt The event loop Go to message queue / In the task queue , Select the procedure that gives priority to meeting the conditions , Put it into the main thread execution stack for execution .（ The event loop ： Waiting for task , An infinite cycle of transitions between executing a task and entering a sleep state waiting for more tasks to be executed ）