GitHub Actions Expression Tester

GitHub Actions expressions use the ${{ }} syntax to dynamically compute values in workflow files. They can reference context variables (github, env, secrets, inputs), call built-in functions (contains, startsWith, toJSON, etc.), and perform boolean/comparison logic to conditionally run steps or set output values.

Common contexts include: github (event_name, ref, sha, actor, repository), env (custom environment variables), inputs (workflow_dispatch inputs), secrets (encrypted secrets), needs (outputs from dependent jobs), and matrix (current matrix values). Each context is a JSON object accessible in expressions.

contains(haystack, needle) returns true if the string or array contains the value. Examples: contains('hello world', 'hello') → true. contains(github.event.labels.*.name, 'bug') → true if any label is named 'bug'. It's case-insensitive for strings.

== is strict equality: github.ref == 'refs/heads/main' is true only for exact match. startsWith(str, prefix) checks if a string begins with the prefix — useful for branch patterns like startsWith(github.ref, 'refs/heads/release/').

Use: if: github.ref == 'refs/heads/main'. For multiple branches: if: contains(fromJSON('["refs/heads/main","refs/heads/develop"]'), github.ref). For PR events targeting main: if: github.event.pull_request.base.ref == 'main'.

fromJSON(value) parses a JSON string into an object. It's commonly used to convert string-encoded matrix values or to compare against arrays. For example: contains(fromJSON('["push","pull_request"]'), github.event_name) checks if the event is a push or pull_request.