Javascript: Correct way to declare functions dynamically

Question

I'm having trouble getting my head around declaring functions dynamically. I tried to ask this question so it was more generic but I really don't know what I'm doing in this case and kept coming back to my code.

I have a json object which contains log levels (as seen in code below) and for each log level I need to create a function so I can call, for example: log.info('my log message');

The problem I'm experiencing is when I call log.info() I can't figure out that 'info' was called for all I know log.error() could of been called because all these functions contain the same dynamic code and I don't know a way to detect what function called this.

I hope that makes sense. I'm writing this for Node.js so my code so far is below. I have a comment saying where my problem function is and where I want the log level to display.

exports.logger = function() {

  // Syslog severity levels
  var levels = {
    emerg: { "code": 0 },
    alert: { "code": 1 },
    crit: { "code": 2 },
    error: { "code": 3 },
    warning: { "code": 4 },
    notice: { "code": 5 },
    info: { "code": 6 },
    debug: { "code": 7 }
  };

  // Loop through each severity level
  for (var severity in levels) {
    // Ensure we're not iterating over a prototype
    if (levels.hasOwnProperty(severity)) {
      // Declare function
      this[severity] = function(message) {
        // Here's the problem: I have no idea what called this function.
        console.log('Log severity level here:', message);
      };    
    }
  }
}

This is how I call it:

var logger = require('log.js');
var log = new (logger.logger)(
  // settings removed for example
);
log.info('test');

Raynos · Accepted Answer · 2012-02-22 18:12:04Z

4

Object.keys(levels).forEach(function (severity) {
  this[severity] = function (message) {
    console.log(severity, message);
  });
}, this);

answered Feb 22, 2012 at 18:12

Raynos

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1 Comment

Sean Bannister Over a year ago

Thanks so much for saving my sanity, was stuck on this for hours :)

Community · Accepted Answer · 2017-05-23 09:59:28Z

1

This is very similar to this recent question: Creating closures within a for loop - am I doing this right?. People, don't create closures in a loop!

  for (var severity in levels) {
    if (levels.hasOwnProperty(severity)) {
      this[severity] = createLogger(severity);
    }
  }

  function createLogger(severity) {
      return function(message) {
        console.log(severity, message);
      }
   }

edited May 23, 2017 at 9:59

CommunityBot

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answered Feb 22, 2012 at 18:08

georg

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