Javascript Question posted in
A very good W3school tutorial can be found here.

Javascript – Undefined throws an exception so how can you test for it?

I have the below code:

    if (typeof profileChart !== undefined) {
        profileChart.destroy();
    } else {
        let profileChart = new Chart(
           pisteElevationProfileChart,
           configPiste
        );
    }

Despite the solution provided here, I still get an error as follows;

script.js?ver=1.0:336 Uncaught ReferenceError: profileChart is not defined

So how can you test for an undefined variable.

Tags:

4

Answers


  1. 0

    Here’s your problem:

    if (typeof profileChart !== undefined) {

    The typeof value is never going to be undefined. You need to check against typeof undefined.

    if (typeof profileChart !== typeof undefined) {

    Explanation:

    typeof profileChart

    This piece of code will evaluate to the string "undefined" if the variable profileChart is not defined.

    "undefined" !== undefined

    This statement will always evaluate to True.

    typeof undefined

    will evaluate to "undefined".

    "undefined" !== "undefined"

    evaluates to False so the else block would be executed when the variable is not defined.

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  2. 0

    Others explained the typeof issue.

    The reference error you get when you do if (profileChart) is because you need to DECLARE the variable before you can test it for falsy values.

    let a;
if (a) // undefined
if (b) // referenceError

    You declare it and wrap it like this

    let profileChart; // This will hold the current instance of the chart

const getProfileChart = (pisteElevationProfileChart, configPiste) => {
  // Destroy the existing chart if it exists
  profileChart?.destroy(); // optional chaining
  // Create a new chart and return it
  return new Chart(pisteElevationProfileChart, configPiste);
};

// before you want to use it

profileChart = getProfileChart(pisteElevationProfileChart, configPiste);
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  3. 0

    The problem can be slightly deeper in the code. What is the line where you declare the variable and what keyword do you use (var, let, or const)? The scope where you are declaring the variable also matters. If, for example, you declare a variable inside an if statement (something like the following:

    if(statement) {
   const profileChart = //...;
}

if (typeof profileChart !== undefined) {
    profileChart.destroy();
}
//...

    ), then the variable will never be accessible (ReferenceError is the error where variable has not been initialized yet and so it can’t be accessed for the type checking). The solution that can work here is:

    let profileChat;
if(statement) {
   profileChart = //...;
}

if (profileChart) {
    profileChart.destroy();
}
//...

    Hope this is well, but it’s better to rearrange this sort of code in favor of the const declaration

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  4. 0

    the issue is that the typeof operator returns a string representing the type of the operand so when checking if a variable is undefined you need to compare the result of typeof with the string undefined

    if (typeof profileChart !== 'undefined') {
    profileChart.destroy();
} else {
    let profileChart = new Chart(
       pisteElevationProfileChart,
       configPiste
    );
}

    best approach would be to declare profileChart with an initial value of undefined outside of your if/else block.

    let profileChart;

// some other code where profileChart might be defined

if (profileChart !== undefined) {
    profileChart.destroy();
} else {
    profileChart = new Chart(
       pisteElevationProfileChart,
       configPiste
    );
}
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