const aspect = require('fit-aspect-ratio')

//calculate a missing width/height
aspect({ratio:'3:4', width:640})
// {width:300, height:480}

aspect({ratio:'widescreen', height:400}) // (2.35:1)
// {width:940, height:400}

//fit to the closest aspect ratio
let res = aspect({
  width: 1280,
  height: 722 // (almost 1280×720 )
})
// { closest:
//    { names: [ '16:9', ... ],
//      description: 'HD video',
//      decimal: 1.77777 },
//   width: 1280,
//   height: 720
// }

// force a specific aspect ratio (assumes landscape)
aspect({ratio:'3:4', height:640, orientation:'portrait'}) //essentially '4:3'
// {width:480, height:640}

<script src="https://unpkg.com/fit-aspect-ratio/builds/fit-aspect-ratio.js"></script>
<script>
  fitAspect({ratio:'A4', height:8.5, orientation:'landscape'})
</script>

you can see the list of aspect-ratios we use here

when fitting to an aspect ratio, height is adjusted, not width. (..scrolling down is easier?) This can be configured in future versions.

ratios are (typically) given as width:height

suppose our computer screen is 4:3, our resolution could be 640x480.

To calculate:

// get a decimal representation
let decimal= 4/3  // 1.3333333

// this can get us our width..
width = 480 * decimal
// 640

// to get height, flip numerator/denominator
let inverse = 1/decimal // or '3/4'
height = 640 * inverse
// 480

this took me a good while...

See also:

• aspect-crop - cropping images based on this library

MIT