How do I write a text file using simple javascript.

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Javascript

var txtFile=new File("CustomerDetails.txt",3);
if(txtFile!=-1)
{
cdata=cname+","+cdob;
fwrite(txtFile,cdata);
fclose(txtFile);
}
it give an error
TypeError : Failed to construct 'File': The provided value cannot be converted to a sequence.

What I have tried:

var txtFile=new File("CustomerDetails.txt",3);
if(txtFile!=-1)
{
cdata=cname+","+cdob;
fwrite(txtFile,cdata);
fclose(txtFile);
}