从javascript回调返回值

Question

I need to pass a value to a function, which contains a modifier method, and then return the modified value.

What's the best way to do this?

I have a read function (which parses content) and a scan function (which looks for a word in the content.

I have the following code so far:

scanContent = function(url){
  read(url, function(err, article, meta) {

      var content = article.title + article.content

      return scan(content)

    }
    return scan(content)
  });

  return scan(content)

}

This obviously doesn't work, but I'm not really sure what the right approach is here.

Answer 1

You need to provide callback function parameter and call it with your result:

function byThree(n, callback) {
    var x = n * 3;
    callback(x);
}
var multiply = function(n, callback) {
    byThree(n, function(result) {
        callback(result);
    });
};

multiply(5, function(result) {
    console.log('result: ' + result);
});

Answer 2

function byThree(n){
    return n * 3;
}
multiply = function(n){
    return byThree(n);
};
console.log("result: "+multiply(5));

Though the following makes more sense:

function multiply(n, m){
    return n * m;
};
console.log("result: "+multiply(5, 3));

And the following makes even more sense:

console.log("result: " + 5 * 3);

Answer 3

If function gets a meaningful result from an inner callback, then it can return a meaningful result only by another callback. You can't trick the program flow.

So, it would be like that:

var scanContent = function(url, cb){
  read(url, function(err, article, meta) {
      var content = article.title + article.content

      return cb(scan(content)); // asume 'scan' is defined elsewhere
    }
  });
};

scanContent(function(text) { console.log(text); });

This could lead to the callback hell . There is Promise from ES6 to avoid that. And generators to make the code even more straightforward.

With Promises (and arrow functions):

var scanContent = function(url){
  // simplified; it's better to always return a promise in the real code
  return read(url).then((err, article) => scan(article.title + article.content));
};

scanContent('example.com').then((text) => console.log(text));