Snapshots & Throws

toMatchSnapshot

Serialize a value and assert it matches a stored snapshot. The first run records the snapshot; later runs compare against it.

expect(renderTable(data)).toMatchSnapshot();         // unnamed
expect(config).toMatchSnapshot("default-config");    // named

Unnamed snapshots within a suite are numbered in order (#1, #2, …); named snapshots use the name you pass. Snapshot identity also includes the file and the suite nesting path, so the same name in different suites stays distinct.

Snapshots are created and updated from the CLI, not by editing files by hand:

ast test --create-snapshots      # record missing snapshots
ast test --overwrite-snapshots   # accept new output as the baseline
ast test --no-snapshot           # skip snapshot assertions this run

See Snapshots for storage format and workflow.

toThrow

Assert that a function throws when called. The value under test must be a () => void callback — toThrow invokes it and checks whether it threw:

expect((): void => {
  throw new Error("boom");
}).toThrow();

expect((): void => parse("not json")).toThrow("invalid input");

toThrow depends on the try-as feature, which rewrites throwable code so exceptions can be intercepted. Enable it in config ("features": ["try-as"]) or per run (--enable try-as). Without it, toThrow reports a failure explaining that try-as is required. See Configuration → features.