You can use a function like this to do the conversion:

function toDegrees (angle) {
  return angle * (180 / Math.PI);
}

Note that functions like sin, cos, and so on do not return angles, they take angles as input. It seems to me that it would be more useful to you to have a function that converts a degree input to radians, like this:

function toRadians (angle) {
  return angle * (Math.PI / 180);
}

which you could use to do something like tan(toRadians(45)).

Math.sin expects the input to be in radian, but you are expecting the result of 90 degree. Convert it to radian, like this

console.log(Math.sin(90 * Math.PI / 180.0));
# 1

As per the wikipedia's Radian to Degree conversion formula,

Angle in Radian = Angle in Degree * Math.PI / 180

Parameters are assumed to be in radians, not degrees.

Try

Math.sin(Math.PI * (30/180));

A comment below notes that pre-computing the ratio π/180 is a good idea. One could add a companion to Math.sin that works on degrees this way:

Math.dsin = function() {
  var piRatio = Math.PI / 180;
  return function dsin(degrees) {
    return Math.sin(degrees * piRatio);
  };
}();

(Some people don't like extending built-in objects, but since one doesn't instantiate Math instances — at least, I don't — this doesn't seem terribly offensive.)