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JavaScript funky formatting

12 October 2011

The task

A number given should be formatted for easy reading by separating digits into triads. Format must be made inline via JavaScript, i.e., frameworks / functions are not welcome.

So let’s take a number like this “12345678.91” and format it to “12 345 678.91”. To be shorter I assign it to a variable and hustled up following line which produced desired format:

> var n = "12345678.91";
> n.split('').reverse().join('').match(/.{1,3}/g).join(' ').split('').reverse().join('').replace(/\s\.|,\./g, '.'));
> "12 345 678.91"

How it’s made

First there is a string that have to be split somehow into triad. This is pretty simple – I’ve just used Regular expression with match and join methods to get the result back into a string using desired digital group separator (space in the case):

> n.match(/.{1,3}/g).join(' ');
> "123 456 78. 91"

The problem here is that RegExp is staring from left to right and produced wrong (from the task’s point of view) result. There is a two spaces where there shouldn’t be: after the dot and in the last triad (having two digits instead of three). First space I’ve eliminated adding regexp replace function.

> n.match(/.{1,3}/g).join(' ').replace(/\.\s/g, '.');
> "123 456 78.91"

Now it seems OK on the right of the dot. But last triad has still two digits. This is because RegExp rules passed to the match function are applied from left to right. So until dot is found it is impossible to predict how triad will be formed. Instead of applying more complicated regexp I’ve decided to reverse numbers before to add group separator.

> n.split('').reverse().join('').match(/.{1,3}/g).join(' ');
> "19. 876 543 21"

This way I’ve passed over the problem with the direction rules have been applied to the input. As the result is reversed at this point, I’ve to switch it back to original order adding same functions, this time on the right end.

> n.split('').reverse().join('').match(/.{1,3}/g).join(' ').split('').reverse().join('');
> "12 345 678 .91"

This is pretty close to desired result nevertheless there is one surplus delimiter before the dot. It might be easily eliminated using replace function

> n.split('').reverse().join('').match(/.{1,3}/g).join(' ').split('').reverse().join('').replace(/\s\./g, '.');
> "12 345 678.91"

That’s it.

Some final thoughts:

  • What if we have a number instead of a string?
var n = 12345678.91;

Applying the long line just composed against a number will produce an error…

Just add one more step in front of all the others:

> n.toFixed(2).split('').reverse().join('').match(/.{1,3}/g).join(' ').split('').reverse().join('').replace(/\s\./g, '.');
> "12 345 678.91"
  • What if a number is requied as result?

Right! Sill another step – multiplication by 1 – at the end.

> n.toFixed(2).split('').reverse().join('').match(/.{1,3}/g).join(' ').split('').reverse().join('').replace(/\s\./g, '.')*1;
> 12 345 678.91
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