If you have any libraries within your workspace, this migration will convert `tslib` peer dependencies to direct dependencies for the libraries.
TypeScript uses the `tslib` package to provide common helper functions used in compiled TypeScript code.
The `tslib` version is also updated to `2.0.0` to support TypeScript 3.9.
Before:
```json
{
"name": "my-lib",
"version": "0.0.1",
"peerDependencies": {
"@angular/common": "^9.0.0",
"@angular/core": "^9.0.0",
"tslib": "^1.12.0"
}
}
```
After:
```json
{
"name": "my-lib",
"version": "0.0.1",
"peerDependencies": {
"@angular/common": "^9.0.0",
"@angular/core": "^9.0.0"
},
"dependencies": {
"tslib": "^2.0.0"
}
}
```
## Why is this migration necessary?
The [`tslib`](https://github.com/Microsoft/tslib) is a runtime library for Typescript.
The version of this library is bound to the version of the TypeScript compiler used to compile a library.
Peer dependencies do not accurately represent this relationship between the runtime and the compiler.
If `tslib` remained declared as a library peer dependency, it would be possible for some Angular workspaces to get into a state where the workspace could not satisfy `tslib` peer dependency requirements for multiple libraries, resulting in build-time or run-time errors.
As of TypeScript 3.9 (used by Angular v10), `tslib` version of 2.x is required to build new applications.
However, older libraries built with previous version of TypeScript and already published to npm might need `tslib` 1.x.
This migration makes it possible for code depending on incompatible versions of the `tslib` runtime library to remain interoperable.
## Do I still need `tslib` as a dependency in my workspace `package.json`?
Yes.
The `tslib` dependency declared in the `package.json` file of the workspace is used to build applications within this workspace, as well as run unit tests for workspace libraries, and is required.
