What causes "expected function to throw an error, but it didn't" in vitest-dev/vitest?

Make the test inputs force the error branch (pass invalid arguments, set failing state).

vitest-dev/vitest · error · AssertionError

expected function to throw an error, but it didn't

Error message

expected function to throw an error, but it didn't

What it means

Thrown by `toThrow`/`toThrowError` when the subject function ran to completion without raising. The matcher invokes the function, and if `isThrow` stays false (and `.not` is not set), it raises `AssertionError`. This is the core failure path for a negative `toThrow`.

Solutions

Make the test inputs force the error branch (pass invalid arguments, set failing state).
If the function legitimately should not throw, remove the `toThrow` assertion or invert with `.not.toThrow()`.
Ensure internal try/catch does not swallow the error before it propagates.

Example fix

// before
expect(() => validate('')).toThrow()
// validate returns silently

// after
expect(() => validate('')).toThrow('required')
// after fixing validate to throw on empty input

Defensive patterns

Strategy: validation

Validate before calling

let threw = false
try { fn() } catch { threw = true }
if (!threw) { throw new Error('setup did not produce a throw') }

Prevention

Drive the test inputs through the branch that throws.
Avoid swallowing errors inside the SUT before they propagate.
Write the test against the real failure path before asserting on it.

When it happens

Trigger: Calling `expect(fn).toThrow()` on a function that does not throw; expecting a specific error but the code path skipped the throw branch (e.g. guarded by a condition that was false).

Common situations: Writing the test before the error path exists; condition guarding the throw was satisfied so the branch was skipped; the function swallowed errors internally with try/catch.

Related errors

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/c6d98b9e195d54d3. Report an issue: GitHub.

Appendix: source

Thrown at packages/expect/src/jest-expect.ts:802

      }
      else {
        let isThrow = false
        try {
          obj()
        }
        catch (err) {
          isThrow = true
          thrown = err
        }

        if (!isThrow && !isNot) {
          const message
            = utils.flag(this, 'message')
              || 'expected function to throw an error, but it didn\'t'
          const error = {
            showDiff: false,
          }
          throw new AssertionError(message, error, utils.flag(this, 'ssfi'))
        }
      }

      if (typeof expected === 'function') {
        const name = expected.name || expected.prototype.constructor.name
        return this.assert(
          thrown && thrown instanceof expected,
          `expected error to be instance of ${name}`,
          `expected error not to be instance of ${name}`,
          expected,
          thrown,
        )
      }

      if (isError(expected)) {
        const equal = jestEquals(thrown, expected, [
          ...customTesters,
          iterableEquality,

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