title: Nodejs : ReferenceError: express is not defined
body: <p>I begin to use Nodejs to create web servers, and for this, I use express module. Then, I intalled it with command : "sudo npm install -g express".
  But after lanched the program, this error occured : </p>
  
  <blockquote>
    <p>"ReferenceError: express is not defined
        at Object. (/home/louis/Documents/Programming/Web/Nodejs/Test/server.js:1:85)
        at Module._compile (module.js:643:30)
        at Object.Module._extensions..js (module.js:654:10)
        at Module.load (module.js:556:32)
        at tryModuleLoad (module.js:499:12)
        at Function.Module._load (module.js:491:3)
        at Function.Module.runMain (module.js:684:10)
        at startup (bootstrap_node.js:187:16)
        at bootstrap_node.js:608:3 "</p>
  </blockquote>
  
  <p>I decided to install it localy but in vain...</p>
  
  <p>Here is the code I use : </p>
  
  <pre><code>let express = require(express);
  let app = express();
  
  console.log("Hello world !");
  
  app.listen(80);
  </code></pre>
  
  <p>Thanks</p>