JavaScript is only capable of handling 53-bit numbers, if you are working with a big number such as Twitter ID, which is using 64-bit number, then you need to find an external library to do that, otherwise, there will be precision lost:

> num = 420938523475451904
420938523475451900
> num = 420938523475451904 + 1
420938523475451900
> num = 420938523475451904 - 1
420938523475451900

Here is one library to use in Node environment, install Big.js:

$ npm install big.js

Load the module:

> BigNum = require('big.js')
{ [Function: Big] DP: 20, RM: 1 }

Use string to create the big number:

> num = BigNum('420938523475451904')
{ s: 1,
  e: 17,
  c: [ 4, 2, 0, 9, 3, 8, 5, 2, 3, 4, 7, 5, 4, 5, 1, 9, 0, 4 ] }
> num.toString()
'420938523475451904'

Perform addition:

> num.plus(1).toString()
'420938523475451905'

Perform substraction:

> num.minus(1).toString()
'420938523475451903'

There are other packages that yet to be tested:

• bignumber.js
• strint.js
• bignumber
• JavaScript bignumber