What you should do is convert every char using charCodeAt function to get the Ascii Code in decimal. Then you can convert it to Binary value using toString(2):

function convert() {
  var output = document.getElementById("ti2");
  var input = document.getElementById("ti1").value;
  output.value = "";
  for (var i = 0; i < input.length; i++) {
      output.value += input[i].charCodeAt(0).toString(2) + " ";
  }
}

<input id="ti1" value ="TEST"/>
<input id="ti2"/>
<button onClick="convert();">Convert!</button>

And here's a fiddle: http://jsfiddle.net/fA24Y/1/

There is a problem with your stringToBinary function. Converting a character to binary only leaves you with the least amount of bits. So you still need to convert it to an 8-bit string.

The other thing is, that you can just get the first 8 digits from your binary and trim your input as you go, like in the following example.

function stringToBinary(input) {
  var characters = input.split('');

  return characters.map(function(char) {
    const binary = char.charCodeAt(0).toString(2)
    const pad = Math.max(8 - binary.length, 0);
    // Just to make sure it is 8 bits long.
    return '0'.repeat(pad) + binary;
  }).join('');
}

function binaryToString(input) {
  let bytesLeft = input;
  let result = '';

  // Check if we have some bytes left
  while (bytesLeft.length) {
    // Get the first digits
    const byte = bytesLeft.substr(0, 8);
    bytesLeft = bytesLeft.substr(8);

    result += String.fromCharCode(parseInt(byte, 2));
  }

  return result;
}

const binary = stringToBinary('test');
console.log({
  binary,
  text: binaryToString(binary),
});

Use String.fromCharCode() and parseInt( , 2) like this:

const binary = `1010011 1110100 1100001 1100011 1101011 
1001111 1110110 1100101 1110010 1100110 
1101100 1101111 1110111`;

const outputStr = binary
  // split string into an array of base 2 (binary) UTF-16 chars
  .split(' ')
  // map every binary char to a UTF-16 number of base 10
  // and then get the string representation of that UTF-16 number
  .map(bin => String.fromCharCode(parseInt(bin, 2)))
  // join the array back to a single string
  .join('');

console.log(outputStr);

• String.fromCharCode(number) will return a String from a UTF-16 char code
• parseInt(binary , 2) will transform a base 2 number string into a base 10 number

Edit:

As the String.fromCharCode() function accepts multiple chars as parameters, you could also use the Spread Operator (...) like so:

const binary = `1010011 1110100 1100001 1100011 1101011 
1001111 1110110 1100101 1110010 1100110 
1101100 1101111 1110111`;

const outputStr = String.fromCharCode(
  ...binary.split(' ').map(bin => parseInt(bin, 2))
)

console.log(outputStr);

Edit 2:

As this answer gets more traffic over time, I am going to also add the solution to do it the other way around too ... just in case:

const str = `StackOverflow`;

const outputStr = str.split('') // split in single chars
  .map(c => c.charCodeAt(0) // get the UTF-16 code (10 base)
             .toString(2)) // transform it back to a string (2 base)
  .join(' ') // make single string from array

console.log(outputStr);

function convertBinary() {
    var output = document.getElementById("outputBinary");
    var input = document.getElementById("inputBinary").value;
    output.value = "";
    for (i=0; i < input.length; i++) {var e=input[i].charCodeAt(0);var s = "";
    do{
        var a =e%2;
        e=(e-a)/2;
        s=a+s;
        }while(e!=0);
        while(s.length<8){s="0"+s;}
        output.value+=s;
    }
}

You welcome my friend

I recently completed an exercise on this using a for loop. Hope it's useful:

function binaryAgent(str) {

var newBin = str.split(" ");
var binCode = [];

for (i = 0; i < newBin.length; i++) {
    binCode.push(String.fromCharCode(parseInt(newBin[i], 2)));
  }
return binCode.join("");
}
binaryAgent('01000001 01110010 01100101 01101110 00100111 01110100');
//translates to "Aren't"

EDIT: after learning more JavaScript, I was able to shortened the solution:

function binaryAgent(str) {

var binString = '';

str.split(' ').map(function(bin) {
    binString += String.fromCharCode(parseInt(bin, 2));
  });
return binString;
}
binaryAgent('01000001 01110010 01100101 01101110 00100111 01110100');
//translates to "Aren't"

If you want to convert the array back to a string use join() (MDN) and for converting a string to an integer use parseInt() (MDN). The second argument of the later is an optional radix.

JavaScript will try to determine, what radix to use, but to be sure you should always add your radix manually. Citing from MDN:

If radix is undefined or 0, JavaScript assumes the following:

• If the input string begins with "0x" or "0X", radix is 16 (hexadecimal).

• If the input string begins with "0", radix is eight (octal). This feature is non-standard, and some implementations deliberately do not support it (instead using the radix 10). For this reason always specify a radix when using parseInt.

• If the input string begins with any other value, the radix is 10 (decimal).

So in your case the following code should work:

var a = '01001011';

var b = parseInt( a.split('').reverse().join(''), 2 );

or just (if you would want to convert the starting string, without the reversal):

var b = parseInt( a, 2 );