Javascript - '...' meaning

Question

I am looking at a open source javascript application, specifically an extension for firefox.

I am seeing this syntax in multiple places that I do not know what it means if anyone can shed some light on this.

such as..

 return (...args)

or...

 console.info(message, ...args.slice(1));

any idea what this '...' does? Is it like getting the third argument in or what? Third argument back? Its hard to try and debug this without being able to understand it.

Answer 1

It's an Ecmascript 6 "rest" parameter. When used as a parameter or argument, it lets you receive or pass an array as individual arguments.

http://ariya.ofilabs.com/2013/03/es6-and-rest-parameter.html

Answer 2

It will unpack an array (args) into a formal argument list. Amongst other things this allows the members of a rest parameter to be passed as a set of formal arguments to another function.

Here's an example:

var stats = function(...numbers) {
    for (var i=0, total = 0, len=numbers.length; i<len; i++) {
        total += numbers[i]; 
    } 
    return {
        average: total / arguments.length,
        max: Math.max(numbers); //spread array into formal params
    } 
} 

stats(5, 6, 8, 5); //{average: 6, max: 8}

Hope this help you understand "..."!