Jest doesn't run — hangs indefinitely

问题

I was originally having this problem with create-react-app, so I did really bare bones setup just of jest:

created new directory
yarn init in that directory
yarn add jest

created new file sum.js:

   function sum(a, b) {
      return a + b;
   }
   module.exports = sum;

created file to test above function

const sum = require('./sum');

test('adds 1 + 2 = 3' , () => {
    expect(sum(1,2)).toBe(3);
});

added to package.json:
```
"scripts": {
"test": "jest"
```
},

But when I run yarn test, I get this:

terry@terry-sharewalker:~/myProjects/test-jest$ yarn jest
yarn run v1.13.0
$ /home/terry/myProjects/test-jest/node_modules/.bin/jest

and nothing happens from there. It just hangs. The same thing happened with running tests from create-react-app. react-scripts test would show, then nothing.

Here's what I got:

Jest "^24.5.0"
Ubuntu 16.04
yarn 1.13.0
watchman 4.9.0
node 10.15.3

I've reinstalled and upgraded everything I can think of, including npm, node, watchman, linuxbrew, yarn. If anybody can help me I'd be forever grateful!!