N/file Module
File.appendLine()
Method Description: Method used to insert a line to the end of a file. This method can be used on text or .csv files. Content held in memory is limited to 10MB. Therefore, each line must be less than 10MB.
Returns: file.File Object
Supported Script Types: Server-side scripts For more information, see SuiteScript 2.0 Script Types.
Governance: None
Module: N/file Module
Since: 2017.1
Search NetSuite - https://system.netsuite.com/app/help/helpcenter.nl?search=File.appendLine(options)

// Code Example 1

//Add additional code

//...

var fileObj = file.load({

    id: 145

});

fileObj.appendLine({

    value: 'hello world'

});

//...

//Add additional code



//SOURCE: https://system.netsuite.com/app/help/helpcenter.nl?fid=section_4769938149.html