"This syntax requires an imported helper but module 'tslib' cannot be found" with ES2015 modules

99
votes

I have demo project I'm about to compile to ES5 with ES2015 modules enabled and tslib used for external TS helpers:

package.json

{
  "name": "foo",
  "scripts": {
    "build": "tsc"
  },
  "dependencies": {
    "tslib": "^1.9.3"
  },
  "devDependencies": {
    "typescript": "^3.1.3"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "target": "es5",
    "module": "es2015",
    "outDir": "./lib",
    "rootDir": "./src",
    "importHelpers": true,
    "strict": true,
    "experimentalDecorators": true
  }
}

src/index.ts

function a(target: any) {
    return target;
}

@a
export class Foo {}

This results in an error:

src/index.ts:5:1 - error TS2354: This syntax requires an imported helper but module 'tslib' cannot be found.

While lib/index.js is correctly compiled:

import * as tslib_1 from "tslib";
function a(target) {
    return target;
}
var Foo = /** @class */ (function () {
    function Foo() {
    }
    Foo = tslib_1.__decorate([
        a
    ], Foo);
    return Foo;
}());
export { Foo };

How can this problem be solved?

12
typescript

12 Answers

120
votes

Noob mistake (which I just made). Try:

npm install tslib

or 

npm i

Personally before signing off on Friday I did a git clean -fxd, but no npm i so all the npm packages were missing. Doh!

20
votes

The problem for me was that the editor was using a different TypeScript version than the project.

To fix that:

  1. Open Command Palette (Cmd+Shift+P on Mac. Focused file must be .ts or .tsx otherwise it won't show the option to change version)
  2. Select "TypeScript: Select TypeScript Version..."
  3. It shows VSCode's TS version and Workspace's (project) one, pick that one

Or click on the version number at the bottom bar if it shows in there:

19
votes

Add below lines to tsconfig.json

"compilerOptions": {
    //...rest parameters

    "baseUrl": "./",
    "paths": {
      "tslib" : ["path/to/node_modules/tslib/tslib.d.ts"]
    },
18
votes

Just updated tslib to latest version, and problem had been fixed. I had installed 1.9.3, and updated to 1.10.0.

6
votes

As the reference states, module resolution is set to Node mode only for "modules": "commonjs", and is set to classic mode for "modules": "es2015":

There are two possible module resolution strategies: Node and Classic. You can use the --moduleResolution flag to specify the module resolution strategy. If not specified, the default is Classic for --module AMD | System | ES2015 or Node otherwise

Since classic mode is unaware of node_modules, the compiler cannot resolve tslib module.

moduleResolution should be set explicitly for ES2015 modules:

...
"module": "es2015",
"moduleResolution": "node",
...
4
votes

My error appeared to be sporadic but likely caused by editing a file in the "node_modules" folder.

I deleted

  • "node_modules" folder
  • "package-lock.json" file

Ran ... "npm install"

It is now working.

NOTE: I tried running "npm install' prior to deleting the files and that did not solve the issue.

3
votes

update the dependencies and devDependencies of tslib in package.json

{
   dependencies:{
     "tslib": "1.10.0",
   },
   devDependencies:{
     "tslib": "1.10.0",
   }
}
2
votes

In my case the error Cannot find module 'tslib' was caused by special builder @nrwl/node:build in my Angular repository (I'm using nrwl/nx monorepo pattern with NodeJS server as one of the applications).

It didn't include tslib in the compilation output. Setting "externalDependencies": "none" in the production build configuration in angular.json solved the issue. Credit goes to @vincastl here: https://github.com/nrwl/nx/issues/2625

Posting it here as it was the first post I found trying to solve this issue, perhaps I'm not alone.

2
votes
npm i -g tslib

This solved my problem. Without -g it didn't work for me.

1
votes

In your project , simply run

npm i tslib

It contains all the typescript helper functions

0
votes

npm install

If you see this error the first time you open a new project or repository, you probably simply haven't installed the app's required modules yet.

In the directory of the app, simply run:

npm install

-1
votes

Add this below line in your .csproj file inside <PropertyGroup>: 

<TypeScriptCompileBlocked>True</TypeScriptCompileBlocked>