Envelope in Javascript

Envelope.js

class Envelope {

constructor(type, strength, fromValue, toValue, sampleCount)
{
    this._generate(type, strength, fromValue, toValue, sampleCount);
}


_generate(type, strength, fromValue, toValue, sampleCount)
{
    let result = new Float32Array(sampleCount);

    let concaveDown = strength < 0;
    if (concaveDown) strength = -strength;

    if (type == "quadratic") {
        for (let i = 0; i < sampleCount; i++) {
            let x = i / sampleCount;
            result[i] = x * x;
        }

    } else if (type == "cubic") {
        for (let i = 0; i < sampleCount; i++) {
            let x = i / sampleCount;
            result[i] = x * x * x;
        }

    } else {
        let x = Math.pow(10.0, -120.0 / 20.0);
        let multiplier = Math.pow(1 / x, 1 / sampleCount);

        for (let i = 0; i < sampleCount; i++) {
            result[i] = x;
            x *= multiplier;
        }
    }

    // Mix in linear
    if (strength < 1) {
        let iStrength = 1.0 - strength;

        for (let i = 0; i < sampleCount; i++) {
            result[i] = strength * result[i] + iStrength * (i/sampleCount);
        }
    }

    // If concaveDown, flip horizontally and vertically
    if (concaveDown) {
        for (let i = 0; i < sampleCount / 2; i++) {
            let i2 = sampleCount - i - 1;
            let tmp = result[i];
            result[i]  = 1.0 - result[i2];
            result[i2] = 1.0 - tmp;
        }
    }

    let toMinusFrom = toValue - fromValue;
    for (let i = 0; i < sampleCount; i++) {
        result[i] = fromValue + result[i] * toMinusFrom;
    }

    this._array = result;
}


apply(sampleArray, startIndex)
{
    if (!sampleArray) return;

    if (startIndex === undefined || (startIndex < 0)) {
        startIndex = 0;
    }

    let myArray  = this._array;
    let endIndex = startIndex + myArray.length;

    if (endIndex > sampleArray.length) {
        endIndex = sampleArray.length;
    }

    let m = 0;
    for (let s = startIndex; s < endIndex; s++) {
        sampleArray[s] *= myArray[m++];
    }
}


}


export default Envelope;