Resize image to exact size by maintaining aspect ratio nodejs

I want to resize image to an exact size by maintaining aspect ratio and filling the empty space with transparency using nodejs.

I am able to do it using canvas. But due to some server os issues I can not use canvas.

I tried with imageMagick and gm. But couldn't find any option like these. Please show me a way to do this.

Thanks in advance.

In ImageMagick, you can resize and fill to the exact size by

convert image -resize WxH -background none -gravity center -extent WxH output

Input:

Here I will make the background black so you can see that if fills it out.

convert lena.jpg -resize 400x300 -background black -gravity center -extent 400x300 lena1.jpg