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Import assignment does not emit import statement in ESNEXT #19500
Description
TypeScript Version: 2.7.0-dev.201xxxxx
Contrary to the documentation (https://www.typescriptlang.org/docs/handbook/modules.html), using import assignment when targeting ES2015/ES6/ESNEXT does NOT emit import statements in the generated JS.
Code
This is my input file:
import m = require('m')
m('123')$ tsc -t ESNEXT index
Expected behavior:
import * as m from "m"
m('123')As described by the documentation
Actual behavior:
m('123')See also this playground (this is 2.4.2, but I tested locally with @latest)
The import is completely omitted
Background info: I am depending on JS libraries which module.exports = someFunction and have the matching declaration file. All this feeds into webpack, which could take care of the module compatibility issues later.
But the import statement is just omitted completely
Activity
mhegazy commentedon Oct 26, 2017
The relevant part is the error message
Import assignment cannot be used when targeting ECMAScript modules. Consider using 'error TS1202: import * as ns from "mod"', 'import {a} from "mod"', 'import d from "mod"', or another module format instead.The behavior of
require("m")is not the same asimport * as m from "m", so it is not clear that there is even a way to transpile this correctly all the time.janv commentedon Oct 26, 2017
You are correct about the error, and I can kinda understand the reasoning here. But I still see two/three issues:
First, the documentation clearly (or am I misunderstanding something there?) describes a different behavior.
Second, the alternatives provided by the error message don't work. It is impossible to
import * as m from 'm', when that module has anexport = function someFunction(){...}, resulting in an error:This makes it impossible to use many real-world CommonJs modules unless there's another way that I'm overlooking?
mhegazy commentedon Oct 26, 2017
Not sure i understand your comment.. just write
import * as m from "m"or use--module commonjs.janv commentedon Oct 26, 2017
I am sorry, I tried to provide a minimal example, I might have omitted a relevant part.
The declaration file for the module I'm importing in questions looks like this:
When I change the import statement as you describe (
import * as m from "m"), typescript gives me the error message quoted in my last post (TS2497).I do not want to use commonjs as an output module format. I can understand that this might be an unsupported use case, my confusion just comes from the fact that according to the documentation, this should work. Search for "native ECMA" on https://www.typescriptlang.org/docs/handbook/modules.html
I have my full test repo here: https://github.com/janv/import-assign-bug.
In the index.ts you find both variants for imports, each with its respective error as a comment after the line. Run
yarn startto build index.tsSorry for this back and forth, thank you for looking into this. I have ways to work around my concrete problem, but I still think this is something worth looking into.
aluanhaddad commentedon Oct 27, 2017
If
"m"is a CommonJS or AMD module, the correct form of the import, and the one which currently typechecks (requires setting the--allowSyntheticDefaultImportscompiler option) isUnfortunately, this will not work at runtime unless you are using SystemJS, a recent version of Webpack, or piping TypeScript's output through Babel.
Resolving this is tracked by #16093
mhegazy commentedon Oct 27, 2017
As Aluan Haddad (@aluanhaddad) mentioned, #16093 is the one you want to track for now.
janv commentedon Oct 30, 2017
Awesome to see you're already onto it :D
Thanks a lot