It treats each element in the array as an array, so '456' is treated as an array with the elements 4,5,6.

A strange workaround I did was to loop through the array and put each element inside its own array, so it would be:

var newArray = [];

 for( var i = 0; i < tempArray.length; i++){
 var arr = [];
 arr.push(tempArray[i]);
newArray.push(arr);
}

And then write the "newArray".

But otherwise if you write row by row, you can put each element inside a bracket, like:

var fast_csv = fastcsv.createWriteStream();
var writeStream = fs.createWriteStream("outputfile.csv");
fast_csv.pipe(writeStream);

for(var i = 0; i < tempArray.length; i++){
    fast_csv.write( [ tempArray[i]  ] );             //each element inside bracket
    }
fast_csv.end();

Well, I know that this question has been asked a long back but just now I got to work with CSV file for creating API with node js. Being a typical programmer I googled "Reading from a file with fast-csv and writing into an array" well something like this but till date, there isn't any proper response for the question hence I decided to answer this.

Well on is async function and hence execution will be paused in main flow and will be resumed only after nonasync function gets executed.

var queryParameter = ()=> new Promise( resolve =>{
   let returnLit = []
   csv.fromPath("<fileName>", {headers : true})
      .on('data',(data)=>{
          returnLit.push(data[<header name>].trim())
      })
      .on('end',()=>{
          resolve(returnLit)
      })
})
var mainList = [];
queryParameter().then((res)=>mainList = res)

If you want to validate something pass argument into queryParameter() and uses the argument in validate method.