ScreenScalar.js

function ScreenScalar(props = { p1:1, p2:1 }) {
    
    const compute = () => this.value = 1 - Object.values(props).reduce((r, v) => r * (1 - v), 1)

    compute()
    
    for (const key of Object.keys(props)) {
        Object.defineProperty(this, key, {
            get: () => props[key],
            set: value => {
                props[key] = value
                compute()
            },
        })
    }
}

"Screen" aka the blend mode

value === 0 if every prop values are 0

usage

const fx = new ScreenScalar({ foo:.5, bar:.5 })
console.log(fx.value) // .75
fx.foo = 0
console.log(fx.value) // .5
fx.bar = 0
console.log(fx.value) // 0