Feed Bazel output to another Bazel rule

Question

I am pretty new to Bazel and have a difficult time figuring out a solution for this:

Say I have this nodejs_binary rule:

nodejs_binary(
    name = "js_scirpt",
    data = [
        "@npm//some_lib",
    ],
    entry_point = ":some_js_script.js",
)

Now I need the output from :js_script be fed to a go_test rule which does something else. The sequence matters: nodejs rule should finishes first and then go_test uses the output.

I think this should be possible by writing a json file from the nodejs_binary to the disk and read it from some_js_script.js, though I cannot control the sequence of executions and I don't know how to pass it to go_test rule. Any idea on how this is possible (or maybe there is a better approach)?

Answer 1

A dependency is the way to make one thing happen before another with Bazel. genrule is the easiest way to run a command and have the output available.

Putting them together looks like this:

genrule(
    name = "run_js_script",
    tools = [
        ":js_script",
    ],
    outs = [
        "something.json",
    ],
    cmd = "$(location :js_script) --output $(location something.json)",
)

go_test(
    data = [
        ":something.json",
    ],
    [name, srcs, deps, etc]
)

The Go code should use runfiles.go to find the path to the file.

Also, the Node code should take a command-line flag for the destination to write its output. If it can't do that, then use shell commands to move the output to $(location something.json).