javascript - Difference between System.import() and import()? - Stack Overflow

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In webpack 1 docs is statement that in webpack 2 will use System.import() for dynamic require:

Luckily, there is a JavaScript API “loader” specification being written to handle the dynamic use case: System.load (or System.import). This API will be the native equivalent to the above require variations.

And during that time all around the web was examples of using this System.import().

---

Before releasing webpack 2, authors decide to change System.import() to import():

add import() as Code Splitting construct. It should be used instead of System.import when possible. System.import will be deprecated in webpack 2 release (removed in webpack 3) as it's behavior is incorrect according to the spec.

This import() is based on tc39/proposal-dynamic-import specification, and you can read more why they made this change here.

---

Can someone explain difference between System.import() and import()?

Despite different name, usage looks the same:

import(modulePath)
  .then(module => module.default())
  .catch(/* ... */);

System.import(modulePath)
  .then(module => module.default())
  .catch(/* ... */);

But in weback 2 doc is: "System.import() behavior is incorrect according to the spec" - so it suggest that there is difference between System.import() and import().

In webpack 1 docs is statement that in webpack 2 will use System.import() for dynamic require:

Luckily, there is a JavaScript API “loader” specification being written to handle the dynamic use case: System.load (or System.import). This API will be the native equivalent to the above require variations.

And during that time all around the web was examples of using this System.import().

---

Before releasing webpack 2, authors decide to change System.import() to import():

add import() as Code Splitting construct. It should be used instead of System.import when possible. System.import will be deprecated in webpack 2 release (removed in webpack 3) as it's behavior is incorrect according to the spec.

This import() is based on tc39/proposal-dynamic-import specification, and you can read more why they made this change here.

---

Can someone explain difference between System.import() and import()?

Despite different name, usage looks the same:

import(modulePath)
  .then(module => module.default())
  .catch(/* ... */);

System.import(modulePath)
  .then(module => module.default())
  .catch(/* ... */);

But in weback 2 doc is: "System.import() behavior is incorrect according to the spec" - so it suggest that there is difference between System.import() and import().

• javascript
• import
• webpack
• ecmascript-6
• webpack-2

Share Improve this question Follow edited Feb 13, 2017 at 10:09 Everettss asked Feb 12, 2017 at 20:03 EverettssEverettss 16.1k99 gold badges7777 silver badges106106 bronze badges 2

• If I am reading the links you have provided right, a very important difference is that import() is aware of the script or module that invoked it, while System.import() is not. If I am right, this means that import('../foo'), i.e. resolution relative to the current module, is possible. Then again I might be badly mistaken, please correct me. – Nikos Paraskevopoulos Commented Feb 12, 2017 at 23:01
• It looks like the difference originated from this github issue – Milan Commented Apr 24, 2017 at 19:26

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1 Answer 1

Sorted by: Reset to default 3

Here's what you're looking for: tc39 Proposal for Import

An actual function

Drafts of the Loader ideas collection have at various times had actual functions (not just function-like syntactic forms) named System.import() or System.loader.import() or similar, which acplish the same use cases.

The biggest problem here, as previously noted by the spec's editors, is how to interpret the specifier argument to these functions. Since these are just functions, which are the same across the entire Realm and do not vary per script or module, the function must interpret its argument the same no matter from where it is called. (Unless something truly weird like stack inspection is implemented.) So likely this runs into similar problems as the document base URL issue for the importModule function above, where relative module specifiers bee a bug farm and mismatch any nearby import declarations.

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