Added utility to calculate EAN barcode type parity bit #56

[uyu423]

Background

EAN bar codes have parity bits by default.
If the parity bit is not correct, the barcode scanner will not recognize it.

For example, for EAN-13 only 12 characters can be specified by the user and the last 13th bit must be calculated as parity bit.

This can be found on Wikipedia. (Https://en.wikipedia.org/wiki/International_Article_Number#Calculation_of_checksum_digit)

Commit Details

• If escpos provides the barcode function, I think you also need the code to calculate the parity bit. So I added the part of the Printer.prototype.barcode function that calculates the parity bit of the EAN type barcode.
• For EAN-13, if code.length is not 12, it is meaningless. So I added an exception for this. EAN-8 also added exception handling when code.length is not 7.
• I created a new ./barcode.js file. The getParityBit (code, type) function is defined inside. I only looked at the EAN type, so I made it easy to add other types.
• reduced the barcode length of ./example/index.js from 8 to 7.
• added a new ./example/barcode.js file.
• I'm using Npm v5 now. package-lock.json file is automatically generated, but it is considered unnecessary for this project and added to .gitignore (I tried not to use the latest JavaScript function as much as possible.)

Testing

• I tried to add the test code but I gave up because I could not check the data inside the buffer. However, I did some tests myself and it worked normally.
• The barcode containing the calculated parity bits was recognized normally by the barcode scanner.

Feedback is needed.
Thanks.

[uyu423]

Why is it closed? It may be a good act to explain why.

[lsongdev]

sorry，im wrong .

[uyu423]

thank you. I will update the README.md soon and pull request.

[lsongdev]

LGTM 👍