Difference between return value when passing array parameter vs rest parameter?

Question

I have a function that returns different values when I pass an array vs. a rest parameter. When I check each with Array.isArray(), they are both arrays. Why is the return value different?

function checkTerm(...terms) {

  var checkSet = ['that','this','else','now'];

  return terms.filter(term => checkSet.indexOf(term) > -1);
}

console.log(checkTerm(['this', 'them', 'else']));

VERSUS

function checkTerm(terms) {

  var checkSet = ['that','this','else','now'];

  return terms.filter(term => checkSet.indexOf(term) > -1);
}

console.log(checkTerm(['this', 'them', 'else']));

Passing Parameter as rest: Expected output = ['this','else'], Actual output = []

Passing Parameter as array: Expected output = ['this','else'], Actual output = ['this','else']

Possible duplicate of Usage of rest parameter and spread operator in javascript — Heretic Monkey
– Heretic Monkey, Commented Oct 8, 2019 at 18:22
Does this answer your question? Spread Syntax vs Rest Parameter in ES2015 / ES6 — Henke
– Henke, Commented Mar 15, 2021 at 14:56
In the first example where I use the rest parameters, I'm putting an array into an array. That is the problem. It can be solved by just accepting the array or accepting the array as [...terms] rather than ...terms. — Ryan O'D
– Ryan O'D, Commented Feb 4, 2022 at 0:00

dee-see · Accepted Answer · 2019-10-08 18:18:46Z

2

In your first example you should have called the function like this:

console.log(checkTerm('this', 'them', 'else'));

The way you invoked it, terms is an array with a single element and that element is ['this', 'them', 'else'].

The "rest" operator is meant to convert separate parameters into an array so you should not pass an array directly to it (unless you want an array of arrays...).

answered Oct 8, 2019 at 18:18

dee-see

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1 Comment

Ryan O'D Over a year ago

Or just accept the array as an array.

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