Your code is adding (concatenating) strings. Are you sure that the code you posted represents your problem? What you have written should work. Be sure in the real code you're not saying:

Copyvar a = '2'; // or something similar

Or if the values are parsed from somewhere, be sure to call parseInt(a, 10) on them before doing the addition, 10 being the radix.

Or as pointed out in the comments the Number function would probably suit your purposes.

Recommended (reduce with default value)

Array.prototype.reduce can be used to iterate through the array, adding the current element value to the sum of the previous element values.

Copyconsole.log(
  [1, 2, 3, 4].reduce((a, b) => a + b, 0)
)
console.log(
  [].reduce((a, b) => a + b, 0)
)

Without default value

You get a TypeError

Copyconsole.log(
  [].reduce((a, b) => a + b)
)

Prior to ES6's arrow functions

Copyconsole.log(
  [1,2,3].reduce(function(acc, val) { return acc + val; }, 0)
)

console.log(
  [].reduce(function(acc, val) { return acc + val; }, 0)
)

Non-number inputs

If non-numbers are possible inputs, you may want to handle that?

Copyconsole.log(
  ["hi", 1, 2, "frog"].reduce((a, b) => a + b)
)

let numOr0 = n => isNaN(n) ? 0 : n

console.log(
  ["hi", 1, 2, "frog"].reduce((a, b) => 
    numOr0(a) + numOr0(b))
)

Speed Optimized

The reduce way is nice as it is easy to write and generally simple to understand, but if you are looking for speed (which is usually not a concern), use a simple for loop.

Copyconst numbers = [1, 2, 3, 4];
let sum = 0;
for (let i = 0; i < numbers.length; i++) {
  sum += numbers[i];
}
console.log(sum);

Non-recommended dangerous eval use

We can use eval to execute a string representation of JavaScript code. Using the Array.prototype.join function to convert the array to a string, we change [1,2,3] into "1+2+3", which evaluates to 6.

Copyconsole.log(
  eval([1,2,3].join('+'))
)

//This way is dangerous if the array is built
// from user input as it may be exploited eg: 

eval([1,"2;alert('Malicious code!')"].join('+'))

Of course displaying an alert isn't the worst thing that could happen. The only reason I have included this is as an answer Ortund's question as I do not think it was clarified.