add a 'pre' version

README.md

@@ -110,6 +110,7 @@ You can either specify specific Julia versions or version ranges. If you specify
 - `'^1.3.0-0'` is a **caret** version range that includes _all_ pre-releases of `1.3.0`. It matches all versions `≥ 1.3.0-` and `< 2.0.0`.
 - `'~1.3.0-0'` is a **tilde** version range that includes _all_ pre-releases of `1.3.0`. It matches all versions `≥ 1.3.0-` and `< 1.4.0`.
 - `'lts'` will install the latest LTS build.
+- `'pre'` will install the latest prerelease build.
 - `'nightly'` will install the latest nightly build.
 - `'1.7-nightly'` will install the latest nightly build for the upcoming 1.7 release. This version will only be available during certain phases of the Julia release cycle.
 

__tests__/installer.test.ts

@@ -61,6 +61,10 @@ describe('version matching tests', () => {
             expect(installer.getJuliaVersion(testVersions, 'lts')).toEqual(installer.getJuliaVersion(testVersions, '1.6'))
             expect(installer.getJuliaVersion(testVersions, 'lts')).toEqual('1.6.7')
         })
+
+        it('pre', () => {
+            expect(installer.getJuliaVersion(testVersions, 'pre')).toEqual('1.11.0-beta1')
+        })
     })
 
     describe('version ranges', () => {

src/installer.ts

@@ -11,6 +11,7 @@ import retry = require('async-retry')
 import * as semver from 'semver'
 
 const LTS_VERSION = '1.6'
+const MAJOR_VERSION = '1' // Could be deduced from versions.json
 
 // Translations between actions input and Julia arch names
 const osMap = {
@@ -87,6 +88,9 @@ export function getJuliaVersion(availableReleases: string[], versionInput: strin
     if (versionInput == 'lts') {
         return getJuliaVersion(availableReleases, LTS_VERSION, false)
     }
+
+    if (versionInput == 'pre') {
+        return getJuliaVersion(availableReleases, MAJOR_VERSION, true)
     }
 
     // Use the highest available version that matches versionInput