eval:

eval() is a global function in JavaScript that evaluates a specified string as JavaScript code and executes it.

eval("alert('this is executed by eval()')");

The eval() function can also call the function and get the result as shown below.

var result;

function Sum(val1, val2)
{
    return val1 + val2;
}

eval("result = Sum(5, 5);");

alert(result);

eval can convert string to JSON object.

var str = '({"firstName":"Bill","lastName":"Gates"})';

var obj = eval(str);

obj.firstName; // Bill 

Recommendation:

It is recommended not to use eval() because it is:

1. Slow
2. Not secure
3. Not readable and maintainable