Is there a function to only assign properties on a destination if the property exists?

Question

This might be a pipe dream, but I'd love a JavaScript (or lodash?) function that does this:

let obj1 = {
  prop1: 'first thing',
  prop2: 'second thing'
};
let obj2 = {
  prop2: 'overridden second thing',
  prop3: 'and another thing'
};

let result = magicalFunction(obj1, obj2);

// result is { prop1: 'first thing', prop2: 'overridden second thing' }

In other words, I need a version of Object.assign that allows me to specify that I want one object's properties to be the "precedent," and that all properties on the source object(s) that do not occur on the precedent should be ignored.

Answer 1

Yes, you can do it - iterate over Object.keys of the source object, and check if there is a property with the same name on the replacer object - replace if there is, and don't if there isn't:

let obj1 = {
  prop1: 'first thing',
  prop2: 'second thing'
};
let obj2 = {
  prop2: 'overridden second thing',
  prop3: 'and another thing'
};

let result = magicalFunction(obj1, obj2);

function magicalFunction(source, replacer) {
  let output = {};
  Object.keys(source).forEach(key => output[key] = replacer[key] || source[key]);
  return output;
}

console.log(result);