json - can't access json parameter

Let us to some basic debugging:

> var data = '{"coord": ... }';
> typeof data
  "string"

So far so good, data is a string.

> JSON.stringify(data);
""{\"coord\": ... }""
> typeof JSON.stringify(data);
  "string"

Apparently JSON.stringify(data) also returns a string. We can see the same value contained in data but now including surrounding quotes (note the double "" at the beginning and the end) and escaped quotes (\").

So what exactly does JSON.stringify do? It will convert any JavaScript value to JSON. Some examples:

> JSON.stringify([]) // array
  "[]"
> JSON.stringify(true) // array
  "true"
> JSON.stringify("foo") // string  
  ""foo""

We can see that passing a string simply produces another JSON encoded string, so that doesn't seem particular helpful. But you are also using JSON.parse, so lets see what effect that has:

> JSON.parse(JSON.stringify(data))
  "{"coord": ... }"
> typeof JSON.parse(JSON.stringify(data))
  "string"

It seems using JSON.parse returns a string again. This shouldn't be too surprising since we are passing a string value to JSON.stringify, which will encode it as a JSON string. Parsing this result must give us back the original value, which was a string. We can verify that easily:

> JSON.parse(JSON.stringify(data)) === data
  true

Yep.

So that doesn't help us converting data to a JavaScript object. Lets just try JSON.parse instead:

> JSON.parse(data)
  Object {coord: Object, weather: Array[2], base: "cmc stations", main: Object, wind: Object…}

That looks much better. Since data contains a JSON encoded object, JSON.parse converts that value to a JavaScript object.

I your example, data is a string, not a javascript object, so you don't need to use JSON.stringify, remove it and it should work:

var data = '{"coord":{"lon":74.34,"lat":31.55},"weather":[{"id":711,"main":"Smoke","description":"smoke","icon":"50d"},{"id":701,"main":"Mist","description":"mist","icon":"50d"}],"base":"cmc stations","main":{"temp":304.6,"pressure":1002,"humidity":62,"temp_min":304.15,"temp_max":305.15},"wind":{"speed":5.1,"deg":130},"clouds":{"all":20},"dt":1466901000,"sys":{"type":1,"id":7133,"message":0.0035,"country":"PK","sunrise":1466899176,"sunset":1466950287},"id":1172451,"name":"Lahore","cod":200}'
    setWeather(data);

function setWeather(data) {
    //NOTE: only parse is needed
    var json = JSON.parse(data);
    alert(json['main']['temp']);
    $('#temp').html(json['main']['temp']);
}

data is a String because of the single quotes , so if you called JSON.stringify(data) will add another double quotes to data , which means in order to convert data to JS object you will need to call JSON.parse(data) two times .

 var obj ='{hello:1}';  //string



var json= JSON.stringify(obj);
console.log(json);               // "\"{hello:1}\""

console.log(JSON.parse(json));   //"{hello:1}"    => still a string

To get your code running correctly by converting data to object , just remove the JSON.stringify()

function setWeather(data) {
    var json = JSON.parse(data); // remove JSON.stringify() => now json is object
    alert(json['main']['temp']);
    $('#temp').html(json['main']['temp']);
}