configuration file log4js.json does not contain a valid configuration

When using the sample configuration file config/log4js.json log4js is unable to load this configuration:

 "appenderList.forEach is not a function" (TypeError: appenderList.forEach is not a function
    at configureAppenders (node_modules\log4js\lib\log4js.js:248:18)
    at configureOnceOff (node_modules\log4js\lib\log4js.js:308:7)
    at Object.configure (node_modules\log4js\lib\log4js.js:371:3)

A valid configuration file looks more like this:

{
    "appenders": [
      {
        "type": "dateFile",
        "filename": "log/access.log",
        "pattern": "-yyyy-MM-dd",
        "category": "http"
      },
      {
        "type": "file",
        "filename": "log/app.log",
        "maxLogSize": 10485760,
        "numBackups": 3
      }
    ]
  }

Inspect config/log4js.json and compare it with the valid configuration example in the issue. Update the sample so log4js can load it without the reported TypeError, then verify that the example application accepts the configuration.