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The problem

You need to sort this JavaScript array in numeric order:

var numbers = [2, 1, 11];

Arrays have a sort method, so lets try using that

var numbers = [2, 1, 11];

console.log(numbers.sort());

The output from running this in Chrome 67 is that the array is now sorted as:

[1, 11, 2]

Which is not what we want, the number 11 is in the wrong place.

The Reason

This result is produced because using the JavaScript sort() without a compare method array causes elements to be sorted by converting them to strings and comparing strings in Unicode code point order.

Apologies! That was a bit opaque so lets take that last paragraph apart. The sort method first converts the numbers to strings so we conceptually have

“2”, “1”, “11”

The  UTF-16 code for each of these are obtained and these are respectively

0032, 0031, 0031 0031

With both “1” and “11”  having a lower UTF-16 code than “2” it should now be slightly clearer why 11 appears before 2.

The Solution

The sort method compares two values to determine which should come first. These two values are usually referred to as a and b. The sort method can have an function, anonymous or named as a parameter. This function is a referred to as a compare function. The compare function allows you as the developer to define the rules for sorting.

The compare function should return a number. That number will be  used to determine which of a or b comes first.

<0

If the compare function returns less than zero a should be shown before b

0

If the compare function returns zero, the a and b should remain in the same order

>0

If the compare function returns greater than zero b should be shown before a

Shown below is an example of anonymous compare function:

function(a, b) {  

  return a - b; 

}

This function can then be passed to the sort() method as shown below. The calls to console.log are instrumentation that you can use to see what is going during each of the three times the compare function is invoked.

 

  var numbers = [2, 1, 11];
  console.log('Original order: ' + numbers);

  numbers.sort(function(a, b) {
   console.log(a + ' - ' + b + ' = ' + (a - b));
   return a - b;
  });   

  console.log('Expected order: ' + numbers);

Pasting this into the Chrome console window will produce the following output:

Stepping through the output, first displayed is the array in it’s original order.

The next two lines shows the results of the compare function and in this invocation the parameters a and b have the values 2 and 1 respectively

2 – 1 = 1 means that the compare function returns a number greater than 0 so that 1(b) will be shown before 2 (a)

The next line shows that the a and b values for the final invocation are now 2 and 11.      2 – 11 = -9 which means that the compare function this time return a number less than zero so that 2(a) will be displayed before 11(b)

The final line shows the results of the sort and now the numbers are sorted as expected.

Acknowledgements

Another plug for this wonderful book which was the inspiration for this page Javascript and JQuery book by Jon Duckett

If your JavaScript  executes but not how you expect it to, one method to find out what is going on is to add code to the script that outputs information to the console as the script executes.

Vanilla Logging

The method that most JavaScript developers default to is to use console.log similar to the screen shot below.

Opening the Chrome DevTools (F12) displays the output for this script as:

It doesn’t take many messages to appear before I reach information overload so I use this method sparingly. To assist in such situations the JavaScript Console object has other methods that can be used to help make categories of logging easy to see.

Console.info(), Console.warn() and Console.error()

In order to differentiate between different classes of logging, you can use the .info, .warn and .error methods of the Console object.

Here is the function updated to use these methods, note that the call to each of these are the same as console.log

And the output from the Chrome DevTools console is:

This output comes from Chrome 67 and note that the first two messages which although use .info() the output is identical to .log(). Unfortunately since Chrome 58 info() and log() are shown identical in the console window. (If you would like to know more this link is a good place to start. The calls to warn and error are shown in red and yellow respectively.

Logging Groups

When reviewing the output from a related set of instructions such as the output from a function, it may help to keep the related information together.  Console.group can be used to help in these situations.

The console output from this function shows, first collapsed:

and then expanded:

Table

Another aid to improve readability, particularly when dealing with a lot of tabular data is to format it into a table using console.table. In example shown, an object is populated with data and then a call is made to console.table passing it the columboEpisodes object.

The output from the Chrome DevTools console is:

Assert

The very best method of aiding readability is to only have a message appear in the console if it breaks the expected condition at that point in the script. Console.assert can be used in such circumstances. If the assertion returns true nothing is shown. This can help reduce the noise of what is shown in the console window.

The output of the script is shown below, note that only one message is shown, the one returns false which is the second one, X is not greater than Y.

Acknowledgements

Javascript & JQUERY Book by Jon Duckett

The MSDN pages on the Console Object