How to check if hex color is "too black"?

I found this WooCommerce Wordpress PHP function (wc_hex_is_light) and I converted to JavaScript. Works fine!

function wc_hex_is_light(color) {
    const hex = color.replace('#', '');
    const c_r = parseInt(hex.substr(0, 2), 16);
    const c_g = parseInt(hex.substr(2, 2), 16);
    const c_b = parseInt(hex.substr(4, 2), 16);
    const brightness = ((c_r * 299) + (c_g * 587) + (c_b * 114)) / 1000;
    return brightness > 155;
}

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@Sliffcak, thanks for comment... To use substring, because substr was deprecated:

function wc_hex_is_light(color) {
    const hex = color.replace('#', '');
    const c_r = parseInt(hex.substring(0, 0 + 2), 16);
    const c_g = parseInt(hex.substring(2, 2 + 2), 16);
    const c_b = parseInt(hex.substring(4, 4 + 2), 16);
    const brightness = ((c_r * 299) + (c_g * 587) + (c_b * 114)) / 1000;
    return brightness > 155;
}

You have to extract the three RGB components individually, and then use a standard formula to convert the resulting RGB values into their perceived brightness.

Assuming a six character colour:

var c = c.substring(1);      // strip #
var rgb = parseInt(c, 16);   // convert rrggbb to decimal
var r = (rgb >> 16) & 0xff;  // extract red
var g = (rgb >>  8) & 0xff;  // extract green
var b = (rgb >>  0) & 0xff;  // extract blue

var luma = 0.2126 * r + 0.7152 * g + 0.0722 * b; // per ITU-R BT.709

if (luma < 40) {
    // pick a different colour
}

EDIT

Since May 2014 tinycolor now has a getBrightness() function, albeit using the CCIR601 weighting factors instead of the ITU-R ones above.

EDIT

The resulting luma value range is 0..255, where 0 is the darkest and 255 is the lightest. Values greater than 128 are considered light by tinycolor. (shamelessly copied from the comments by @pau.moreno and @Alnitak)