Is javascript synchronous?

JavaScript is synchronous; JavaScript is always synchronous and single-threaded, and is only asynchronous in the sense that ajax calls can be made; synchronous means that if a block of JavaScript code is being executed on the page, then on the page No other JavaScript will currently be executed.

The operating environment of this tutorial: Windows 10 system, JavaScript version 1.8.5, Dell G3 computer.

Is javascript synchronous?

JavaScript is always synchronous and single-threaded.

If you are executing a block of JavaScript code on a page, no other JavaScript will currently be executed on that page.

JavaScript is asynchronous only in the sense that it can make Ajax calls.

The Ajax call will stop execution and other code will be able to execute until the call returns (successful or otherwise), at which point the callback will run synchronously. No other code will be run at this time. It does not interrupt any other code currently running.

JavaScript timer runs with this same type of callback.

Describing JavaScript as asynchronous can be misleading. To be precise, JavaScript is synchronous and single-threaded with various callback mechanisms.

jQuery has an option on Ajax calls to make them synchronized (with the async: false option). Beginners may use it incorrectly because it allows the use of a more traditional programming model that they may be more accustomed to. The reason this is problematic is that this option will block all

JavaScript on the page until completed, including all event handlers and timers.

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Javascript language is a single-threaded language

Single-threading and asynchronous cannot be features of a language at the same time. js chose to be a single-threaded language, so it itself cannot be asynchronous. Because the browser only assigns one main thread to js to execute those functions, only one can be executed at a time. These tasks form a task queue waiting to be executed, but some tasks on the front end are very time-consuming, such as network requests and timing. If the processor and event listener are queued up to wait for execution like other tasks, the execution efficiency will be very low, and even cause the page to freeze.

How to achieve asynchronous effect in JavaScript single thread

So, the browser opens up another thread for these time-consuming tasks, mainly Including http request thread, browser timing trigger, browser event trigger thread. The host environment (browser) enables js to have asynchronous attributes through event-driven methods.

All tasks can be divided into two types, one is synchronous task (synchronous) and the other is asynchronous task (asynchronous). Synchronous tasks refer to tasks queued for execution on the main thread. The next task can only be executed after the previous task is executed; asynchronous tasks refer to tasks that do not enter the main thread but enter the "task queue". Task, only when the main thread task is completed and the "task queue" starts to notify the main thread and request the execution of the task, the task will enter the main thread for execution.

The CPU is too busy, so forget it, but many times the CPU is idle because the IO devices (input and output devices) are very slow (for example, Ajax operations read data from the network) and have to wait. After the result comes out, continue the execution.

Specifically, the asynchronous running mechanism is as follows:

All synchronous tasks are executed on the main thread, forming an execution context stack.

Besides the main thread, there is also a "task queue". As long as the asynchronous task has running results, an event is placed in the "task queue".

Once all synchronous tasks in the "execution stack" are completed, the system will read the "task queue", and those asynchronous tasks will enter the execution stack and start execution.

The main thread continues to repeat the third step above.

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