How to test for object passed as ...rest argument to a function

Question

I am practicing using ...rest operator to pass arguments to functions. I have run into an issue that I'm unable to resolve for when passing an object to the function.

Here is the function:

function sum(...args){
  if (args.length === 1 && Array.isArray(args[0])) {
    args = args[0];
  }
  return args.reduce((a,b) => a + b);
}

console.log(sum([5,4,3,2,1])); // Returns 15
console.log(sum(5,4,3,2,1)); // Returns 15
console.log(sum({a:5,b:4,c:3,d:2,e:1})); // Returns {a:5,b:4,c:3,d:2,e:1}

I'm stuck at how to test for and handle the last use case.

Answer 1

Convert your object to an array with Object.values, passing an object instead of an array is an overkill:

function sum(...args){
  if (args.length === 1 && Array.isArray(args[0])) {
    args = args[0];
  }
  return args.reduce((a,b) => a + b);
}

console.log(sum([5,4,3,2,1])); // Returns 15
console.log(sum(5,4,3,2,1)); // Returns 15
console.log(sum(Object.values({a:5,b:4,c:3,d:2,e:1}))) //

Also you could possibly improve your function with Array::flat(). That way it will behave the same as Array::concat(). I like that.

function sum(...args){
  return args.flat().reduce((a,b) => a + b);
}

console.log(sum([5,4,3,2,1])); // Returns 15
console.log(sum(5,4,3,2,1)); // Returns 15
console.log(sum(Object.values({a:5,b:4,c:3,d:2,e:1}))) // Returns 15
console.log(sum(
  [5,4,3,2,1],
  5,4,3,2,1,
  Object.values({a:5,b:4,c:3,d:2,e:1})
)); // Returns 45

I wouldn't convert an object to an array automatically inside sum but if you need:

function sum(...args){
  return args
    .map(arg => typeof arg === 'object' && !Array.isArray(arg) ? Object.values(arg) : arg)
    .flat()
    .reduce((a,b) => a + b);
}

console.log(sum([5,4,3,2,1])); // Returns 15
console.log(sum(5,4,3,2,1)); // Returns 15
console.log(sum({a:5,b:4,c:3,d:2,e:1})) // Returns 15
console.log(sum(
  [5,4,3,2,1],
  5,4,3,2,1,
  {a:5,b:4,c:3,d:2,e:1}
)); // Returns 45