setInterval function cannot use outer variable

Question

Why is prevColor always undefined?

This code should log a new color (actualColor) and the previous color (prevColor). However I cannot save into prevColor from inside the setInterval function. Where is the bug? I don´t think it is a context problem. But I´m not sure. There´s no this inside...

Can you tell me how I can save the value of actualColor in prevColor from inside the setInterval function?

var actualColor;
var prevColor;


// do some user action and change actualColor


setInterval(function () {
    // only log the color if it differs from prevColor
    if (actualColor != prevColor) {
        console.log("actualColor: " + actualColor + ", prevColor: " + prevColor);
    }
    prevColor = actualColor;
}, 100);

Console:

actualColor: acff06, prevColor: undefined

Possible duplicate of How do I return the response from an asynchronous call? — Whymarrh
– Whymarrh, Commented Dec 1, 2015 at 22:07
I left this part out in this code snippet. It would have been too long. There is a comment inside the code. — wurstbrotrest
– wurstbrotrest, Commented Dec 1, 2015 at 22:12
If i set a number to prevColor in line 2 the console will always log this value. But no updated Color — wurstbrotrest
– wurstbrotrest, Commented Dec 1, 2015 at 22:15

Matiu Carr · Accepted Answer · 2015-12-01 23:03:18Z

1

I think it must be your context -I made a simple webpage with all the bits you have above, and it works fine -even without setting initial values on the variables:

I placed your code in a script tag in the HEAD, and added

<input
    type="text"
    id="actualColor"
    />
<input
    type="button"
    onclick = "actualColor = document.getElementById('actualColor').value;"
    value = "change colour" />

to provide a way of changing actualColor in the webpage (rather than using the console)

answered Dec 1, 2015 at 23:03

Matiu Carr

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Comments

void · Accepted Answer · 2015-12-01 22:10:51Z

0

You need to initialize the value of the prevColor, otherwise first time it will be undefined

Just do

var actualColor;
var prevColor = "";


// do some user action and change actualColor


setInterval(function () {
    // only log the color if it differs from prevColor
    if (actualColor != prevColor) {
        console.log("actualColor: " + actualColor + ", prevColor: " + prevColor);
    }
    prevColor = actualColor;
}, 100);

answered Dec 1, 2015 at 22:10

void

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