[geometry-1] Methods of immutable classes should return immutable instances.

[trusktr]

For example, DOMMatrixReadOnly.multiply should return DOMMatrixReadOnly instead of DOMMatrix.

This makes the API awkward to use when choosing immutability:

const mat1 = new DOMMatrixReadOnly(...)
const mat2 = new DOMMatrixReadOnly(...)
const mat3 = new DOMMatrixReadOnly(mat1.multiply(mat2).toFloat64Array())

Not only is this awkward, but it wastefully creates 4 matrices and an unnecessary typed array, instead of only 3 matrices, because mat1.multiply(mat2) returns a DOMMatrix, so we need to call toFloat64Array to pass the array to DOMMatrixReadOnly to create the immutable isntance.

Furthermore, the way the APIs currently work prevents people from using the APIs for subclasses. 🥲

class MyMatrix extends DOMMatrix {
  specialNewMethod() {
    const newMat = this.rotateAxisAngle(...)
    // do something with newMat
    return newMat
  }
}

Now the user's new specialNewMethod will erroneously return a DOMMatrix instance instead of a MyMatrix instance, forcing the user to make the code more expensive again like this:

class MyMatrix extends DOMMatrix {
  specialNewMethod() {
    const newMat = this.rotateAxisAngle(...)
    // do something with newMat
    return new this.constructor(newMat.toFloat64Array())
  }
}

Instead, the ideal API would be like this:

const mat1 = new DOMMatrixReadOnly(...)
const mat2 = new DOMMatrixReadOnly(...)
const mat3 = mat1.multiply(mat2)

If the browser's implementation of multiply (and other immutable methods) were to use the equivalent of new this.constructor(), making subclasses would be simpler, and immutability would also be preserved as expected.

Related:

• [geometry] return DOMPoint or DOMPointReadOnly accordingly, from matrixTransform method? #275