constant-aspect-ratio.js

var adjustAspectRatio = function (rectObj, wRetio, hRetio) {
    if ((typeof rectObj.width === 'undefined' || rectObj.width === null)
        || (typeof rectObj.height === 'undefined' || rectObj.height === null)) {
        console.error('Error : Object is not rect!');
        return;
    }
    if (rectObj.width > rectObj.height * wRetio / hRetio) {
        rectObj.width = rectObj.height * wRetio / hRetio;
    }
    else if (rectObj.height > rectObj.width * hRetio / wRetio) {
        rectObj.height = rectObj.width * hRetio / wRetio
    }
    return rectObj;
};


// --------
// TestCode
// --------
var aspectWidth = 64, aspectHeight = 39;
var rectOverWidth = { width: 1000, height: 390 };
var rectOverHeight = { width: 640, height: 640 };
var rectError = { hoge: '', foo: '' };
var rectNoneWidth = { height: 100 };
var rectNoneHeight = { width: 100 };

console.log(adjustAspectRatio(rectOverWidth, aspectWidth, aspectHeight));   // {width:640, heigt:390}
console.log(adjustAspectRatio(rectOverHeight, aspectWidth, aspectHeight));  // {width:640, heigt:390}
console.log(adjustAspectRatio(rectError, aspectWidth, aspectHeight));       // 'Error : Object is not rect!'
console.log(adjustAspectRatio(rectNoneWidth, aspectWidth, aspectHeight));   // 'Error : Object is not rect!'
console.log(adjustAspectRatio(rectNoneHeight, aspectWidth, aspectHeight));  // 'Error : Object is not rect!'