The fourslash tests are an integration level testing suite. By this point they have a very large API surface, and tend to cover a lot of the "user-facing" aspects of TypeScript. E.g. things which an IDE might have an interest in knowing.
gulp runtests will run all the fourslash tests eventually. Or gulp runtests -i --tests=[filename] should speed
things up if you only want to see those specific changes.
Fourslash automatically generates mocha tests based on files you put inside /tests/cases/fourslash the code
for this lives in /src/testRunner/fourslashRunner.ts. This class is instantiated in
/src/testRunner/runner.ts.
From here the main work all lives in /src/harness/foudslash.ts where we'll be spending the rest of this
section. The initial entry point is runFourSlashTest but the work is in runFourSlashTestContent.
This function first creates a virtual fs, uses [parseTestData][6] to fill up the virtual fs. parseTestData:
- Loops through every line in the test file
- If the line starts with
////then it starts piping the text into a new string which represents the current file - If the line starts with
//then check whether it's a special case variable (likeFilename) - if it's not then it will get passed as though it were a TSConfig setting.
This isn't eval-ing the code, so the tests under are ignored. Here's an example test file:
/// <reference path="fourslash.ts" />
// #29038
// @allowJs: true
// @checkJs: true
// @Filename: /node_modules/moment.d.ts
////declare function moment(): void;
////export = moment;
// @Filename: /b.js
////[|moment;|]
goTo.file("/b.js");
verify.importFixAtPosition([
`import moment from "moment";
moment;`
]);