title: How can I solve "ReferenceError: expect is not defined" error message?
body: <p>I am trying to test Javascript with mocha. I've this snippet of code:</p>
  
  <pre><code>describe('Array', function() {
      describe('indexOf()', function() {
          it("dovrebbe tornare -1 quando l'elemento non è presente", function() {
              expect([1,2,3].indexOf(4)).to.equal(-1)
          })
      })
  })
  </code></pre>
  
  <p>and a <code>test/array.js</code> file. Mocha was installed with</p>
  
  <pre><code>$ npm install -g mocha
  </code></pre>
  
  <p>When I run</p>
  
  <pre><code>$ mocha
  </code></pre>
  
  <p>I get this error:</p>
  
  <pre><code>$ mocha
  ․ 
  
  0 passing (5ms)
  1 failing
  
  1) Array indexOf() dovrebbe tornare -1 quando l'elemento non è presente:
   ReferenceError: expect is not defined
    at Context.&lt;anonymous&gt; (/Users/simonegentili/Desktop/Javascipt Best Practice/test/array.js:4:4)
    at Test.Runnable.run (/usr/local/lib/node_modules/mocha/lib/runnable.js:211:32)
    at Runner.runTest (/usr/local/lib/node_modules/mocha/lib/runner.js:358:10)
    at /usr/local/lib/node_modules/mocha/lib/runner.js:404:12
    at next (/usr/local/lib/node_modules/mocha/lib/runner.js:284:14)
    at /usr/local/lib/node_modules/mocha/lib/runner.js:293:7
    at next (/usr/local/lib/node_modules/mocha/lib/runner.js:237:23)
    at Object._onImmediate (/usr/local/lib/node_modules/mocha/lib/runner.js:261:5)
    at processImmediate [as _immediateCallback] (timers.js:317:15)
  </code></pre>