error-logs.js

// definitions

const toLines = logs => logs.split("\n");
const notEmpty = line => line.trim();
const toParts = line => line.split(" ");

const toLogEntry = parts => ({
  path: parts[parts.length - 2],
  status: parseInt(parts[parts.length - 1])
});

const isError = entry => entry.status >= 400;

const incErrorCount = (errors, entry) =>
  Object.assign(errors, { [entry.path]: (errors[entry.path] || 0) + 1 });

const collectErrors = (logs, errorCounts) =>
  toLines(logs)
    .filter(notEmpty)
    .map(toParts)
    .map(toLogEntry)
    .filter(isError)
    .reduce(incErrorCount, errorCounts);

// usage

let = errorCounts = {};
const getErrorCount = (path) => errorCounts[path] || 0;

const sample = `
8:00:00 192.1.1.2 GET /index 200
8:00:01 192.1.1.1 GET /index 201
8:00:01 192.1.1.2 GET /msgs/ 401
8:00:01 192.1.1.2 GET /msgs/ 404
8:00:02 127.1.1.1 GET /index 200

8:00:03 127.1.1.3 POST /msgs/send 200
8:00:03 127.1.1.2 POST /msgs/send 500
8:00:03 127.1.1.2 POST /msgs/send 501
8:00:03 127.1.1.2 POST /msgs/send 502
8:00:03 127.1.1.2 POST /auth/login 500
8:00:03 127.1.1.2 POST /auth/login 504
8:00:03 127.1.1.2 GET /msgs/receive 403
`
const sample2 = `
8:00:01 192.1.1.2 GET /msgs/ 401
`

errorCounts = collectErrors(sample, errorCounts);
errorCounts = collectErrors(sample2, errorCounts);

console.log(getErrorCount("/msgs/")); // expected: 3