Why Do ES2015 Import Declarations Fail in Firefox, Despite Being at the Top Level?

ES2015 Import Declarations Not Working in Firefox, Even at Top-Level

Users trying to utilize ES2015 import declarations in their HTML files for browser-based JavaScript modules are encountering a peculiar error in Firefox 46. The error message, "SyntaxError: import declarations may only appear at top level of a module," seems puzzling given that the import statement is placed at the very beginning of the script file.

Firefox's Requirement for Explicit Module Declaration

The error in Firefox stems from the fact that ES2015 module support is not enabled by default in the browser. To use modules with import and export, you must explicitly indicate that you are using a module by adding the "type=module" attribute to the script tag that imports the module.

For example, the HTML file in the question should be modified as follows:

<code class="html"><script src="t1.js" type="module"></script></code>

This modification clarifies to Firefox that the script is a module, allowing the import declaration to function properly.

Browser Support for ES2015 Modules

ES2015 module support in browsers is still relatively new and varies across different browsers and versions. Here is a table summarizing browser support for ES2015 import/export capabilities:

If you are using an older browser or a version without built-in module support, you may need to enable experimental flags to activate this feature. The specific flags and instructions vary depending on the browser.

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