the teacher said that the spread operator is on the right side of the "=" This seems like a really bad way of describing things. The "rest" operator appears inside of parameter lists (like in your example), or inside a destructuring declaration (like let [a, ...b] = [1, 2, 3]). It occurs where variables are being created. The "spread" operator is used when you want to "spread"/flatten the data you have into another structure. That can occur inside argument lists (some_func(...data)), or inside of something like an array ([1, 2, 3, ...someData, 4, 5, 6]). In both cases, the placement of the = is irrelevant. More on reddit.com

Console log is a good example of the rest one for anyone struggling to think of it. You've probably written console.log(1, 2, 3, 4) etc in the past, that is to say you can pass any number of arguments into the function and it will be ok with it More on reddit.com

I think its very confusing More on reddit.com

I wouldn't recommend naming the function parameter of a reduce previous.

function sum(...args) {
  return args.reduce((previous, current)=> {
    return previous + current; 
  });
}

The first parameter is the accumulator. total or previousTotal or anything that suggests that we're building on it may be better.

previous suggests that we're comparing to the previous item in the array, and examples that use it can make reduce functions even more complicated to understand than they need to be.

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