如何访问 JavaScript 匿名函数中的变量

Question

How can I access variable C?

test = function(){
    a = 1;
    b = 2;
    c = a+b;
    return c
}
console.log(c)

Returns:

Uncaught ReferenceError: c is not defined

EDIT: Please disregard above question

To elaborate more on this. I am new to JavaScript and am stuck on a block of code that I can't get to work. I tried to simplify the problem I was having but I think that may have backfired...

Here is the full code that I am having trouble with. I need to access the variables inside of getAPIData.onload , But am getting "undefined" What is the best way to access these values?

getData();

function getData(){
  var activeMachines = [41, 44, 45]
  var dict = {};
  let data = activeMachines.reduce((obj, machineID) => {
    var getAPIData = new XMLHttpRequest();
    var url = 'http://127.0.0.1:8000/processes/apidata/' + machineID + '/';
    getAPIData.open('GET', url);
    getAPIData.send();
    getAPIData.onload = function(){
      var APIData = JSON.parse(getAPIData.responseText);
      dict['temp' + machineID] = APIData[0].tempData;
      dict['humid' + machineID] = APIData[0].humidData;
      timeValue = String((APIData[0].dateTime));
      dict['time' + machineID] = new Date(timeValue);
    }
    temp = dict['temp'+ machineID];
    humidity = dict['humid'+ machineID];
    time = dict['time'+ machineID];
    obj['machine_'+ machineID] = {temp, humidity, time}
    return obj
  }, {})
  console.log(data);
}

This returns the data dictionary, but all of the values are undefined. Any help is appreciated. I will read more into JavaScript scope as others have suggested.

Answer 1

c is a global variable that is created by calling the anonymous function assigned to the global variable test . However, you never call the anonymous function assigned to test , therefore the variable never gets created.

So, the obvious solution is to just call the anonymous function assigned to test :

test();

console.log(c);
// 3

However, this is a really bad idea. Global variables are evil, evil, evil. They tie together disparate parts of your program into a giant tangled mess: since they are accessible from everywhere, you have to assume that they may be accessed from everywhere. And changing them in one place may change the behavior of some completely unrelated code in a completely different part of your code.

Actually, it is even worse: since they are global , changing them in one place may not only change the behavior of some completely unrelated code in a completely different part of your code, it may even change the behavior of some completely unrelated code in a completely separate part of someone else's code!

The only way to avoid this, would be to invent unique names for every global variable, otherwise they are going to clash. In fact, you would need to invent a unique name for every global variable that was ever written, is written, and will be written by every programmer in the universe for all eternity!

But actually, there is no need for global variables here at all, since you actually return the value of c in your anonymous function. You can make those variables local to your function and simply use the return value. In fact, you don't even need variables at all, since you never "vary" them, you can make them constants instead.

Note: in modern ECMAScript, you should always prefer const s (unless you have a very good reason to use a let ) and never use var or even worse (like you are doing here) implicit global variable definition. So, your code, in modern ECMAScript, should look like this:

const test = function () {
    const a = 1, 
        b = 2, 
        c = a + b;
    return c;
}

console.log(test());

Also, since you don't give your function a name and don't rely on dynamic scoping of this , you can use arrow function literal syntax:

const test = () => {
    const a = 1, 
        b = 2, 
        c = a + b;
    return c;
}

console.log(test());

or just give your function a name:

function test() {
    const a = 1, 
        b = 2, 
        c = a + b;
    return c;
}

console.log(test());

Answer 2

the variables AB and C are of use locally within the function, but it returns the value of C, with just calling the function returns the value, as it says - charlietfl console.log(test()) , you can call it that way or:

 var c = test ();
    alert(c) or console.log (c);

Now you can use the name varibale "c" outside the function.