chroma-js/src/ops/alpha.js

JavaScript library for color conversions

const Color = require('../Color');
const {type} = require('../utils');

Color.prototype.alpha = function(a, mutate=false) {
    if (a !== undefined && type(a) === 'number') {
        if (mutate) {
            this._rgb[3] = a;
            return this;
        }
        return new Color([this._rgb[0], this._rgb[1], this._rgb[2], a], 'rgb');
    }
    return this._rgb[3];
}