build tests in jest code example

Example 1: what is jest

Jest is a JavaScript testing framework maintained by Facebook, Inc. with a
focus on simplicity.

It works with projects using: Babel, TypeScript, Node.js, React, Angular and
Vue.js.

It aims to work out of the box and config free.

Example 2: jest

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