How can I check if a file contains a string or a variable in JavaScript?

You can not open files client side with javascript.

You can do it with node.js though on the server side.

fs.readFile(FILE_LOCATION, function (err, data) {
  if (err) throw err;
  if(data.indexOf('search string') >= 0){
   console.log(data) //Do Things
  }
});

Newer versions of node.js (>= 6.0.0) have the includes function, which searches for a match in a string.

fs.readFile(FILE_LOCATION, function (err, data) {
  if (err) throw err;
  if(data.includes('search string')){
   console.log(data)
  }
});