javascript - New es6 syntax for importing commonjs / amd modules i.e. `import foo = require('foo')`

Question

Previously I could do:

import foo = require('foo');

But now that TypeScript (1.5) supports es6 module syntax, what is the correct way to achieve the same in ES6 module syntax.

Answer 1

The correct way is to continue using the old import syntax. The new import syntax is for ES modules only, the old import syntax is for pre-ES6 modules. The two are distinct, intentionally so. import * as foo from 'foo' imports all the properties of the module 'foo', it does not import the default value as foo.

From the designer of the feature:

• An export default declaration always declares an exported member named default and is always emitted as an assignment to exports.default. In other words, export default consistently has ES module semantics. For compatibility with Babel we could optionally emit an __esModule marker when a module has a default export, but we wouldn't actually use that marker for anything.
• An export = declaration, which substitutes a different entity to be exported in place of the module itself, is always emitted as an assignment to module.exports. It is an error to have other exports in a module that uses export =. This is the existing TypeScript behavior.
• A module that uses export = to export another module (be that an internal or external module) can be imported using the new ES6 constructs. In particular, the convenient destructuring imports can be used with such modules. The pattern of using export = to export another module is common in .d.ts files that provide a CommonJS/AMD view of an internal module (e.g. angular.d.ts).
• A module that uses export = to export a non-module entity in place of the module itself must be imported using the existing import x = require("foo") syntax as is the case today.

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2016 update: The TypeScript compiler at some point started allowing import * as foo from 'legacy-module-foo' to get the default import of a legacy module in certain circumstances. This is a violation of the ES6 specification (§15.2.1.16, “The value "*" indicates that the import request is for the target module’s namespace object.”).

When legacy modules you import in this manner are updated to ES6 modules, the “default” imports for those modules will stop working (because * as foo imports are supposed to be importing namespace objects), which may be extremely confusing if you don’t know that doing this is a TypeScript/SystemJS hack. It is also possible that a future TypeScript realignment to the ES specification will cause them to break.

As such, you should probably prefer to continue to use the legacy import syntax described above to load legacy modules to avoid confusing yourself and other developers working on your code about how ES6 namespace imports work, and to avoid confusing breaking changes.