Can't understand: "message": content && content.toString() || null;

Question

In a node.js application, I found a function (which is a monkey-patch for the http.ServerResponse class) having two parameters passed to it:

content: I suppose this is a message string (or JSON) which will be the response message to the client.
status: a number.

This the function code:

http.ServerResponse.prototype.respond = function (content, status) {

  if ('undefined' == typeof status) { // only one parameter found
    if ('number' == typeof content || !isNaN(parseInt(content))) { // usage "respond(status)"
      status = parseInt(content);
      content = undefined;
    } else { // usage "respond(content)"
      status = 200;
    }
  }
  if (status != 200) { // error
    content = {
      "code":    status,
      "status":  http.STATUS_CODES[status],
      "message": content && content.toString() || null
    };
  }
  if ('object' != typeof content) { // wrap content if necessary
    content = {"result":content};
  }
  // respond with JSON data
  this.send(JSON.stringify(content)+"\n", status);
};

I couldn't understand the meaning of the value affected to the content's message property in this line:

"message": content && content.toString() || null

As the '&&' is a logical operator so content and content.toString() must be interpreted as binaries (true/false). What does it mean to convert a String to a Binary in javascript? (Is it that when it's not null or undefined so it will be as true?)

I have made a test like this:

//content = {var1:'val1', var2:'val2'};

content = "some content here";

/*console.log('content.toString() : ' + content.toString()); // [object Object]

console.log('JSON.stringify() : ' + JSON.stringify(content));

*/

console.log('content == true' + content == true); // this will display : false
console.log('content.toString() == true' + content.toString() == true); // false too.. 

console.log('content && content.toString() : ' + content && content.toString()); // this will display nothing
console.log('(content && content.toString()) == true' + (content && content.toString()) == true); // false


var message =  content && content.toString() || null; //


console.log('message : ' + message);

And as it's shown:

content == true // returns false
content.toString() == true // returns false too

I just assume that the value affected to the "message" tests if a content exists and converts it to String but I don't understand the logic behind it.

*"As the '&&' is a logical operator so content and content.toString() must be interpreted as binaries (true/false)"* Just read the documentation, you totally misunderstand what's `&&`. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators#Description — Denys Séguret, Oct 22 '15 at 08:59
The `&&` is described here: [Javascript AND operator with assignment](http://stackoverflow.com/questions/3163407/javascript-and-operator-with-assignment) and `||` here: [JavaScript OR (||) variable assignment explanation](http://stackoverflow.com/questions/2100758/javascript-or-variable-assignment-explanation) — t.niese, Oct 22 '15 at 09:00
It's pretty much the equivalent of `content ? content.toString() : null` (which to me is a lot more readable in this instance). — James Thorpe, Oct 22 '15 at 09:00
Your third test (the one with `content && content.toString()`) should display `some content here` string. I can't see how it could display nothing. — Leonid Beschastny, Oct 22 '15 at 09:07
@LeonidBeschastny I tried it, it does exactly what you expected. — Jamiec, Oct 22 '15 at 09:17

Can't understand: "message": content && content.toString() || null;

0 Answers0