The problem is the way you're using forEach(), as it will always return undefined. You're probably looking for the map() method, which returns a new array:
var tifOptions = Object.keys(tifs).map(function(key) {
return <option value={key}>{tifs[key]}</option>
});
If you still want to use forEach(), you'd have to do something like this:
var tifOptions = [];
Object.keys(tifs).forEach(function(key) {
tifOptions.push(<option value={key}>{tifs[key]}</option>);
});
Update:
If you're writing ES6, you can accomplish the same thing a bit neater using an arrow function:
const tifOptions = Object.keys(tifs).map(key =>
<option value={key}>{tifs[key]}</option>
)
Here's a fiddle showing all options mentioned above: https://jsfiddle.net/fs7sagep/