Algorithm for phase adjustment between waves with two different frequencies

index.js

/** 
 * @protected {number} lastFrequency => In hertz, initialized as 0
 * @protected {number} sampleRate  => Audio output sample rate (e.g.: 8000 for 8KHz, 16000 for 16KHz, 44100 for 44.1KHz, 48000 for 48KHz and so on)
 * @protected {number} frame => Current accumulative sample index 
 * @protected {number} offset => Accumulative phase, initialized as 0 
 *  
 * @param {number} frequency in hertz 
 * 
 * @returns {number} sample (-1 <= sample <= 1)
*/

const PI2 = Math.PI * 2;

let theta_A = (PI2 * frequency * frame / sampleRate) % PI2;

if(frequency != lastFrequency){
    let theta_B = (PI2 * lastFrequency * frame / sampleRate) % PI2;

    let cycle_A = theta_A / PI2;
    let cycle_B = theta_B / PI2;

    offset = (offset + (cycle_A - cycle_B) * PI2) % PI2;
}

let sample = Math.sin(theta_A - offset);

lastFrequency = frequency;