Here's some basic JavaScript code for creating a snow effect on a webpage

// create a canvas element and set its dimensions
const canvas = document.createElement("canvas");
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
document.body.appendChild(canvas);

// get the 2D context of the canvas
const ctx = canvas.getContext("2d");

// create an array to hold snowflakes
const snowflakes = [];

// define the snowflake class
class Snowflake {
  constructor() {
    this.x = Math.random() * canvas.width;
    this.y = Math.random() * canvas.height;
    this.radius = Math.random() * 3 + 1;
    this.speed = Math.random() * 3 + 1;
  }
  
  // update the snowflake's position
  update() {
    this.y += this.speed;
    if (this.y > canvas.height) {
      this.y = 0;
      this.x = Math.random() * canvas.width;
    }
  }
  
  // draw the snowflake on the canvas
  draw() {
    ctx.beginPath();
    ctx.arc(this.x, this.y, this.radius, 0, Math.PI * 2);
    ctx.fillStyle = "white";
    ctx.fill();
  }
}

// create a function to generate new snowflakes
function generateSnowflakes(num) {
  for (let i = 0; i < num; i++) {
    snowflakes.push(new Snowflake());
  }
}

// call the function to generate 50 snowflakes
generateSnowflakes(50);

// create a function to animate the snowflakes
function animateSnow() {
  // clear the canvas
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  
  // update and draw each snowflake
  snowflakes.forEach((snowflake) => {
    snowflake.update();
    snowflake.draw();
  });
  
  // request the next frame of the animation
  requestAnimationFrame(animateSnow);
}

// start the animation
animateSnow();

This code creates a canvas element, generates a specified number of snowflakes as instances of a Snowflake class, and animates their movement and appearance on the canvas. The generateSnowflakes() function can be called with a different number to change the amount of snowflakes created, and the Snowflake class can be modified to change the snowflake appearance or behavior.