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Flow Test Suite

This directory contains the test suite for Flow. Each subdirectory represents a test case with source files and expected outputs.

For all commands described below, they must be run from the project root (flow).

Quick Start

OSS Build (using Make)

# Build Flow
make

# Run all tests
./runtests.sh bin/flow

# Run a specific test
./runtests.sh -t <test_name> bin/flow

# Example: run the records test
./runtests.sh -t records bin/flow

Meta Internal Development (using Buck)

./runtests.sh -t mytest $(buck2 build //flow:flow --show-full-output | awk '{print $2}')

Common Commands

Run tests matching a pattern

./runtests.sh <flow_binary> -f "^record"  # All tests starting with "record"

List tests without running them

./runtests.sh -l <flow_binary>

Update expected output after intentional changes

./runtests.sh -t <test_name> -r <flow_binary>

This re-records the test output. Use this when you've intentionally changed Flow's behavior and need to update the .exp files.

Test Structure

Each test directory contains:

  • Source files (.js, .flow, etc.) - The JavaScript/Flow code being tested
  • .exp file - Expected type errors and their format (human-readable)
  • .out file - Generated output from the last test run (auto-generated, not checked in)
  • .err file - Error output if the test failed (auto-generated)
  • test.sh (optional) - Custom test script for tests with special requirements

Tips

  • Always build Flow before running tests - Stale binaries lead to confusing results
  • Use -t for single tests during development - Faster iteration

Adding New Tests

Each test is a directory under tests/ containing source files and a .flowconfig.

.flowconfig

Add all=true to the .flowconfig so that // @flow pragmas are not required in test .js files:

[options]
all=true

Declaring variables with specific types

Use declare const to create a variable of a given type without needing a runtime value:

declare const x: number;
declare const y: Map<string, number>;

Do not use declare var.

To reuse variable names, wrap declarations in blocks to scope them:

{
  declare const x: number;
  // use x as number here
}
{
  declare const x: string;
  // use x as string here
}

Casting syntax

Use as for type casts:

x as T;

Expecting errors

Add an end-of-line // ERROR comment on lines where you expect Flow to report an error:

declare const x: number;
x as string; // ERROR

Capturing a variable's type in the snapshot

To record the inferred type of a variable in the test snapshot, cast it to empty to force an error. The error output will include the variable's type:

x as empty; // ERROR

Common Issues

"No such file or directory" for flow binary

Make sure you're using the full absolute path to the flow binary:

# Wrong (relative path may fail)
./runtests.sh -t mytest ../bin/flow

# Correct (absolute path)
./runtests.sh -t mytest /full/path/to/bin/flow

# Or use command substitution (Meta only)
./runtests.sh -t mytest $(buck2 build //flow:flow --show-full-output | awk '{print $2}')