How Does JavaScript Script Execution Order Work in Web Pages?

Execution Order of JavaScript Scripts in a Web Page

In web development, understanding the loading and execution order of JavaScript scripts is crucial. Various methods exist to include JavaScript in a web page, each with its implications for loading and execution.

JS Loading and Execution Order

By default, scripts are executed in the order they appear in the page, regardless of whether they are external or inline. However, deferring or asynchronously loading scripts can alter this behavior.

• defer: Defers execution until the DOM is fully parsed. All deferred scripts execute in the order they were encountered, after page load.
• async: Loads in parallel and executes as soon as possible, potentially disrupting execution order and event handlers.

Inserted Script Execution Order

Dynamically inserting scripts via DOM manipulation can affect execution order depending on the browser. In Firefox, inserted scripts with an "async" attribute execute immediately, while those without it execute like deferred scripts.

Example: Inline Scripts and External Script Insertion

Consider a scenario with head scripts for initialization and a dynamically appended external script in the body.

• Initial Head Scripts: Execute first, as they appear earlier in the page.
• Inline Body Script Appending External Script: Executes after the head scripts.
• Externally Appended Script: Its execution depends on browser behavior and the async/defer attributes.

Browser Differences

The execution order of dynamically inserted scripts can vary across browsers. Some browsers, like newer versions of Firefox, automatically set inserted scripts to "async," while others handle them synchronously.

Module Scripts

Modern browsers support JavaScript module loading using "

In summary, the loading and execution order of JavaScript scripts is influenced by their location in the page, the presence of defer/async attributes, and browser-specific behaviors for inserted scripts and module scripts. Understanding these factors is essential for ensuring proper script execution and avoiding potential issues.

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