'operator' !== '-' is a string comparison. It's important to note that operator cannot be equated to symbols like -,/,*.

While it has not been verified, it seems like the comparison should be made with the token variable rather than the operator itself.

if (token === '+' || token === '-' || token === '/' || token === '*') {
  // Rest of code
  if (token === '+') {
} else if (token === '-') {
    // more code here
  }
// additional code
}

The issue with the else being flagged as an "unexpected token" arises from incorrect usage of { and } in the if/else if/else structure.

The accurate syntax should be:

if (condition) {
  statement1
} else if (condition) {
  statement2
} else {
  statement3
}

...where the curly braces { and } are optional for a single statement but mandatory for multiple ones within a block.

In your code, each if and else if block spans two lines, necessitating encapsulation within { and }.

Furthermore, the semicolon at the end of this line:

if ( token === '+' || token === '-' || token === '/' || token === '*' ) ;

signifies no execution if the if condition is met, despite commencing the next line with a {.

To rectify bracket misplacements, your revised segment should resemble:

function(inputLine) {
  console.log(inputLine);
  var tokens = inputline.split(' ');
  for (var i = 0; i < tokens.length; i++) {
    const token = tokens[i];
    var tokenCategory = 'operand';
    
    if (token === '+' || token === '-' || token === '/' || token === '*') {
      tokenCategory = 'operator';
      
      var B = ArrayStack.pop();
      var A = ArrayStack.pop();
      
      if (token === '+') {
        var answer = A + B;
        ArrayStack.push(answer);

      } else if (token === '-') {
        var answer = A - B;
        ArrayStack.push(answer);

      } else if (token === '/') {
        var answer = A / B;
        ArrayStack.push(answer);

      } else if (token === '*') {
        var answer = A * B;
        ArrayStack.push(answer);
      }
    } else {
      ArrayStack.push(token);
      console.log(ArrayStack);
    }
  }
}

(You can also place the curly brackets on separate lines per preference.)

Moreover, ensure you're comparing variable token rather than the string 'operator' where needed.

Don't forget to include basic if-else brackets in your code. As mentioned by @brk, make sure you are comparing two strings: 'operator' and '+', '-'. Below is the revised code for you to review.

'use strict';

var ArrayStack = require('./ArrayStack');
const fs = require('fs');
const readline = require('readline');

new ArrayStack();
var readStream = fs.createReadStream('Input_File.txt', 'utf8');
var rl = readline.createInterface({input: readStream});
rl.on('line', function(inputLine) {
    console.log(inputLine);
    var tokens = inputLine.split(' ');
    
    for(var i = 0; i < tokens.length; i++) {
        const token = tokens[i];
        var tokenCategory = 'operand';
        
        if(token === '+' || token === '-' || token === '/' || token === '*') {
            tokenCategory = 'operator';
            var B = ArrayStack.pop();
            var A = ArrayStack.pop();
            
            if(token === '+') {
                var answer = A + B;
                ArrayStack.push(answer);
            }
            elseif(token === '-') {
                var answer = A - B;
                ArrayStack.push(answer);
            }
            elseif(token === '/') {
                var answer = A / B ;
                ArrayStack.push(answer);
            }
            elseif(token === '*') {
                var answer = A * B;
                ArrayStack.push(answer);
            }//end of sub conditional statement
        } 
        else {
            ArrayStack.push(token);
            console.log(ArrayStack);
        }
    }//end of for loop
}); //end of function  

rl.on('close', function() {
    console.log('File now closed.');
}); // end function

I have made some changes to your code based on my understanding. Make sure to test this updated version.

var ArrayStack = require('./ArrayStack');
const fs = require('fs');
const readline = require('readline');

var myStack;
var readStream = fs.createReadStream('Input_File.txt', 'utf8');
var rl = readline.createInterface({input: readStream});
rl.on('line', function(inputLine) {
    console.log(inputLine);
    var tokens = inputLine.split(' ');

    ArrayStack.init(tokens, function(err, data) {
        if (err) {
            console.error('Error received');
        } else {
            myStack = data;
            console.log('pushed data ' + data.first());
        }
    });

    for (var i = 0; i < tokens.length; i++) {
        const token = tokens[i];
        var tokenCategory = 'operand';

        if (token === '+' || token === '-' || token === '/' || token === '*') {
            tokenCategory = 'operator';
            var B = myStack.pop();
            var A = myStack.pop();

            if (token === '+') {
                var answer = A + B;
                myStack.push(answer);
            } elseif (token === '-') {
                var answer = A - B;
                myStack.push(answer);
            } elseif (token === '/') {
                var answer = A / B;
                myStack.push(answer);
            } elseif (token === '*') {
                var answer = A * B;
                myStack.push(answer);
            } //end of sub conditional statement
        } else {
            myStack.push(token);
            console.log(myStack);
        }
    }//end of for loop
}); //end of function

rl.on('close', function() {
    console.log('File now closed.');
}); // end function