tasks.json

{
    "version": "0.1.0",
    "command": "npm",
    "options": {
        "cwd": "${workspaceRoot}"
    },
    "isShellCommand": true,
    "args": [
        "run",
        "watch"
    ],
    "showOutput": "always",
    "isBackground": true,
    "problemMatcher": {
        "fileLocation": "absolute",
        "owner": "ocaml",
        "watching": {
            "activeOnStart": false,
            "beginsPattern": ">>>> Start compiling",
            "endsPattern": ">>>> Finish compiling"
        },
        "pattern": [
            {
                "regexp": "^File \"(.*)\", line (\\d+)(?:, characters (\\d+)-(\\d+))?:$",
                "file": 1,
                "line": 2,
                "column": 3,
                "endColumn": 4
            },
            {
                "regexp": "^(?:(?:Parse\\s+)?(Warning|[Ee]rror)(?:\\s+\\d+)?:)?\\s+(.*)$",
                "severity": 1,
                "message": 2,
                "loop": true
            }
        ]
    }
}