local variable in if statement javascript

Question

var request = require('request');
var cheerio = require('request');
var eval = "5555";

request('http://google.com', function (error, response, body) {
    if (!error && response.statusCode == 200) {
        var keys = body.split(",");
        eval = keys[0]; // Think keys[0] is 7777
// and no problem in if statement, it must be excuted.
    }
})

console.log("evalue :"+eval);
// result ---> 5555

Why the result is 5555? not 7777? I think eval is global value, and if statement will update eval value to 7777, and I think the result will be 7777 but I realize it doesn't. Can anyone tell me why. Sorry to similar question.

`request` is async. So, your console.log fires *before* the callback. — Davin Tryon, Dec 10 '15 at 17:17
`require` isn't part of plain JavaScript. What else does this need to be tagged with? — j08691, Dec 10 '15 at 17:18

score 2 · Answer 1 · answered Dec 10 '15 at 17:17

The callback in request is asynchronous.

Therefore, when you call console.log, it is before the request callback is executed. Try logging in the callback itself

request('http://google.com', function (error, response, body) {
    if (!error && response.statusCode == 200) {
        var keys = body.split(",");
        eval = keys[0]; // Think keys[0] is 7777
        console.log("evalue :"+eval);
    }
})

score 1 · Answer 2 · answered Dec 10 '15 at 17:17

You are doing an asynchronous task. The callback function does not necessarily runs before the console.log.

This is the nature of javascript non-blocking IO. When you need to do an IO operation, you can pass a callback, when the IO finishes, it will invoke your callback, meanwhile CPU is free to do other procedures in your JS file.

score 0 · Answer 3 · answered Dec 10 '15 at 17:17

Because console.log is executed before above request's response has arrived since this is an asynchronous execution. Try putting the console.log inside that anonymous function to get the value you want.

request('http://google.com', function (error, response, body) {
    if (!error && response.statusCode == 200) {
        var keys = body.split(",");
        eval = keys[0]; // Think keys[0] is 7777
        console.log(eval);
    }
})

local variable in if statement javascript

3 Answers3