JavaScript中使用插值实现图像渐变的步骤如下：

1. 创建canvas元素，并设置其宽度、高度等属性。

<canvas id="canvas"></canvas>

1. 获取canvas元素的2D上下文对象，用于绘图。

const canvas = document.getElementById('canvas');
const context = canvas.getContext('2d');

1. 创建图像对象，并在对应方法的回调函数中进行渐变处理。

const image = new Image();
image.src = 'path/to/image.png'; // 图片路径
image.onload = function() { // 图片加载完毕后执行
  const gradient = context.createLinearGradient(0, 0, canvas.width, canvas.height); // 创建线性渐变
  gradient.addColorStop(0, 'red'); // 设置每个颜色停止点
  gradient.addColorStop(0.5, 'yellow');
  gradient.addColorStop(1, 'green');
  context.drawImage(image, 0, 0); // 在画布上绘制原图像
  context.fillStyle = gradient; // 设置填充颜色为渐变色
  context.fillRect(0, 0, canvas.width, canvas.height); // 填充渐变色
}

1. 将最终渐变处理后的图像绘制在画布上。

const result = new Image();
result.src = canvas.toDataURL(); // 获取渐变图像的base64编码
document.body.appendChild(result); // 将渐变图像添加到body元素中

以下是两个示例：

示例一：线性渐变

<canvas id="canvas1"></canvas>

const canvas1 = document.getElementById('canvas1');
const context1 = canvas1.getContext('2d');

const image1 = new Image();
image1.src = 'path/to/image.png';
image1.onload = function() {
  const gradient1 = context1.createLinearGradient(0, 0, canvas1.width, canvas1.height);
  gradient1.addColorStop(0, 'red');
  gradient1.addColorStop(0.5, 'yellow');
  gradient1.addColorStop(1, 'green');
  context1.drawImage(image1, 0, 0);
  context1.fillStyle = gradient1;
  context1.fillRect(0, 0, canvas1.width, canvas1.height);
}

const result1 = new Image();
result1.src = canvas1.toDataURL();
document.body.appendChild(result1);

示例二：径向渐变

<canvas id="canvas2"></canvas>

const canvas2 = document.getElementById('canvas2');
const context2 = canvas2.getContext('2d');

const image2 = new Image();
image2.src = 'path/to/image.png';
image2.onload = function() {
  const gradient2 = context2.createRadialGradient(canvas2.width / 2, canvas2.height / 2, 0, canvas2.width / 2, canvas2.height / 2, 200);
  gradient2.addColorStop(0, 'red');
  gradient2.addColorStop(1, 'green');
  context2.drawImage(image2, 0, 0);
  context2.fillStyle = gradient2;
  context2.fillRect(0, 0, canvas2.width, canvas2.height);
}

const result2 = new Image();
result2.src = canvas2.toDataURL();
document.body.appendChild(result2);