You can set a variable to something with the eval() function by using something similar to the below code:

var x = 10;
var y = 20;
var a = eval("x * y") + "<br>";
var b = eval("2 + 2") + "<br>";
var c = eval("x + 17") + "<br>";

var res = a + b + c;

The result, stored in the variable res, will be:

200427

The use of eval is strongly discouraged. See the Remarks section for details.