ramda进化功能示例

Question

From Ramda Repl:

var tomato  = {firstName: '  Tomato ', data: {elapsed: 100, remaining: 1400}, id:123};

Why does this work:

var transformations = {
  firstName: ()=>'Potato'
};
// => {"data": {"elapsed": 100, "remaining": 1400}, "firstName": "Potato", "id": 123}

But this doesnt:

var transformations = {
  firstName:'Potato'
};
//=>{"data": {"elapsed": 100, "remaining": 1400}, "firstName": "  Tomato ", "id": 123}

R.evolve(transformations, tomato);

Answer 1

R.evolve

Creates a new object by recursively evolving a shallow copy of object , according to the transformation functions . All non-primitive properties are copied by reference.

A transformation function will not be invoked if its corresponding key does not exist in the evolved object.

In short, the transformation must be a function .

Why does this work:

 var transformations = { firstName: ()=>'Potato' }; 

Because ()=>'Potato' is a function

But this doesnt:

 var transformations = { firstName:'Potato' }; 

Because 'Potato' is a string , not a function .

In such a case when the provided transformation is not a function, the original value.

Here's the source code for evolve . I bolded the code path your example takes to arrive at the output

module.exports = _curry2(function evolve(transformations, object) {
  var result = {};
  var transformation, key, type;
  for (key in object) {
    transformation = transformations[key];
    type = typeof transformation;
                     ? transformation(object[key])
                :  ? evolve(transformation, object[key])
                                                      : ;
  }
  return result;
});

Answer 2

@naomik's explained why - but if for some reason you need to use evolve, you could do:

{
    firstName: R.always('Potato')
}

It's worth remembering that the argument given to the transform is the current value, and if the key doesn't exist it won't add anything.