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13.3.4 Client-Side JavaScript Timeline

13.3.4 Client-Side JavaScript Timeline


We’ve already seen that JavaScript programs begin in a script execution phase and then transition to an event-handling phase. This section explains the timeline of JavaScript program execution in more detail.

  1. The web browser creates a Document object and begins parsing the web page, adding Element objects and Text nodes to the document as it parses HTML elements and their textual content. The document.readyState property has the value “loading” at this stage.
  2. When the HTML parser encounterselements that have neither the async nor defer attributes, it adds those elements to the document and then executes the inline or external script. These scripts are executed synchronously, and the parser pauses while the script downloads (if necessary) and runs. Scripts like these can use document.write() to insert text into the input stream. That text will become part of the document when the parser resumes. Synchronous scripts often simply define functions and register event handlers for later use, but they can traverse and manipulate the document tree as it exists when they run. That is, synchronous scripts can see their ownelement and document content that comes before it.
  3. When the parser encounters aelement that has the async attribute set, it begins downloading the script text and continues parsing the document. The script will be executed as soon as possible after it has downloaded, but the parser does not stop and wait for it to download. Asynchronous scripts must not use the

2. Some browsers guard against denial-of-service attacks and accidental infinite loops by prompting the user if a script or event handler takes too long to run. This gives the user the chance to abort a runaway script.

13.3 Execution of JavaScript Programs | 323

document.write() method. They can see their ownelement and all document elements that come before it, and may or may not have access to additional document content.

  1. When the document is completely parsed, the document.readyState property changes to “interactive”.
  2. Any scripts that had the deferattribute set are executed, in the order in which they appeared in the document. Async scripts may also be executed at this time. Deferred scripts have access to the complete document tree and must not use the document.write() method.
  3. The browser fires a DOMContentLoaded event on the Document object. This marks the transition from synchronous script execution phase to the asynchronous event-driven phase of program execution. Note, however, that there may still be async scripts that have not yet executed at this point.
  4. The document is completely parsed at this point, but the browser may still be waiting for additional content, such as images, to load. When all such content finishes loading, and when all async scripts have loaded and executed, the document.readyState property changes to “complete” and the web browser fires a load event on the Window object.
  5. From this point on, event handlers are invoked asynchronously in response to user input events, network events, timer expirations, and so on.

This is an idealized timeline and all browsers do not support all of its details. The load event is universally supported: all browsers fire it, and it is the most common technique for determining that the document is completely loaded and ready to manipulate. The DOMContentLoaded event fires before the load event and is supported by all current browsers except IE. The document.readyStateproperty is implemented by most browsers at the time of this writing, but the values of this property differ slightly from browser to browser. The defer attribute is supported by all current versions of IE, but it is only now being implemented by other browsers. Support for the async attribute is not yet common at the time of this writing, but asynchronous script execution via the technique shown in Example 13-4 is supported by all current browsers. (Notice, though, that the ability to dynamically load scripts with functions like loadasync() blurs the boundary between the script loading and event-driven phases of program execution.)

This timeline does not specify when the document becomes visible to the user or when the web browser must start responding to user input events. Those are implementation details. For very long documents or very slow network connections, it is theoretically possible that a web browser will render part of a document and allow the user to start interacting with it before all the scripts have executed. In that case, user input events might be fired before the event-driven phase of program execution has formally started.

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