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

Question

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?

Answer 1

Try something like:

npm install tslib --save-dev

Or, to fix noob mistake (which I just made):

npm i

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

Answer 2

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:

Answer 3

Add below lines to tsconfig.json

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

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

Answer 4

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

Answer 5

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",
...

Answer 6

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.

Answer 7

In my case, tslib was already installed and i was seeing this error only for 1 component while everything else was working fine - build and functionality. I just restarted my vscode and it vanished. So, try that if the error is still there after you have done things which others have suggested.

Answer 8

update the dependencies and devDependencies of tslib in package.json

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

Answer 9

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.

Answer 10

npm i -g tslib

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

Answer 11

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, run:

npm install

Answer 12

In my case removing or setting the importHelpers compiler option to false resolved the issue.

{
  "compilerOptions": {
    ...
    "importHelpers": false, // Or remove this
    ...
  }
}

Answer 13

In your project, simply run

npm i tslib 

It contains all the typescript helper functions

Answer 14

在你的项目中执行： npm install --force

Answer 15

upgrading tslib version 2.00.0

in package.json it should be "tslib": "^2.0.0"

Answer 16

I had a case, where the only fix was to add this manually in package.json :

"tslib": "^2.3.1",

and run npm i afterwards.

Answer 17

I faced this issue with jest (+vuejs and a monorepo structure). All the app works fine, but the unit tests.

So 2 things has to be done to make them work:

1. in tsconfig.json :

    "compilerOptions": {
      "paths": {
        "tslib" : ["path/to/node_modules/tslib/tslib.d.ts"]
    }

2. in jest.config.js :

    moduleNameMapper: {
      'tslib': '<rootDir>/path/to/node_modules/tslib/tslib.js'
    }

Answer 18

I was facing the same problem and it got resolved using following command!

npm install -D tslib@latest

Answer 19

You have to remove from "compilerOptions": { "importHelpers": true, } on tsconfig.json

Answer 20

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

<TypeScriptCompileBlocked>True</TypeScriptCompileBlocked>