Convert hexadecimal color string to integer in javascript

Question

I have written a function in javascript to convert an integer form of colour from db to hex colour format.But I am unable to convert hex colour string to int form. Also parseInt(color.substr(1), 16) is giving different result.

<html>
<body>
<button onclick="myFunction()">Try it</button>
<p id="test"></p>
<script>
function myFunction() {

    var color="#ff0000";    
    var num = -65536;
    var alphalessHexString =getHexColor(num);
    var n = alphalessHexString+"</br>";
    var ques="i want a function to convert "+color +"  to  "+num;
    document.getElementById("test").innerHTML = n+ques;
}

function getHexColor(number){
    return "#"+((number)>>>0).toString(16).slice(-6);
}

</script>

</body>
</html>

@JaromandaX result is not as expected after using parseInt(color.substr(1), 16) function/ — avinash
– avinash, Commented Apr 6, 2018 at 5:59
var color="#ff0000"; console.log(parseInt(color.substr(1), 16)) -> outputs 16711680 ... why is that not expected? — Jaromanda X
– Jaromanda X, Commented Apr 6, 2018 at 6:06
@JaromandaX, he's probably looking at the signs too, where first bit is the sign. — Pranav Totla
– Pranav Totla, Commented Apr 6, 2018 at 6:08
I want function opposite to getHexColor.For example i am passing -65536 to getHexColor and getting #ff0000,what i want is function which can return me -65536 when i pass #ff0000 to it. — avinash
– avinash, Commented Apr 6, 2018 at 6:10

Jaromanda X · Accepted Answer · 2018-04-06 06:29:10Z

9

If you want a signed 24 bit value, the function is

function colorToSigned24Bit(s) {
    return (parseInt(s.substr(1), 16) << 8) / 256;
}
console.log(colorToSigned24Bit('#ff0000'))

Explanation:

                                                                                 signed 32
                                                                                 bit number
                               value    32 bit binary                            in decimal
-------------------------  ---------    ---------------------------------------  ----------
parseInt(s.substr(1), 16)   16711680    0000 0000 1111 1111 0000 0000 0000 0000    16711680
16711680 << 8             4278190090    1111 1111 0000 0000 0000 0000 0000 0000   -16777216
-16777216 / 256               -65536    1111 1111 1111 1111 0000 0000 0000 0000      -65536

edited Apr 6, 2018 at 6:29

answered Apr 6, 2018 at 6:17

Jaromanda X

1

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2 Comments

avinash Over a year ago

Thanks buddy that's what i was looking for .

Tim Sylvester Over a year ago

The example depends on sign extension to end up with an alpha value of FF. If you do this instead with "#00FF00" you'll get 65280 = 0000FF00 = 0000 0000 0000 0000 1111 1111 0000 0000 . If the result ends up transparent, you probably want to add ...|0xFF000000.

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