Rest Parameters:

In ES5, when you don't know how many paramters will be passed in, you can use arguments:

let sum = function(){

    let result = 0;

    for(let i = 0; i < arguments.length; i++){

        result += arguments[i];

    }

    return result;

}

let result = sum(1,2,3);

In ES6, you can use Rest params:

let sum = function(...numbers){

    let result = 0;

    for(let i = 0; i < numbers.length; i++){

        result += numbers[i];

    }

    return result;

};
describe("rest paramters", function(){

    it("is like varargs", function(){

        let doWork = function(name, ...numbers){

            let result = 0;

            numbers.forEach(function(n){

                result += n;

            });

            return result;

        };

        let result = doWork("Scott", 1,,2,3);

        expect(result).toBe(6);

    });

});

...Spread:

It looks the same as Rest Parameters, Spread uses to spread an array.

it("should sum up", function(){

    let doWork = function(x,y,z){

        return x+y+z;

    };

    expect(doWork(...[1,2,3])).toBe(6);

});
<!DOCTYPE html>

<html>

  <head>

    <script data-require="traceur@*" data-semver="0.0.0-20140302" src="https://traceur-compiler.googlecode.com/git/bin/traceur.js"></script>

    <script>

     traceur.options.experimental = true;

    </script>

    <script data-require="traceur@*" data-semver="0.0.0-20140302" src="https://traceur-compiler.googlecode.com/git/src/bootstrap.js"></script>

    <link rel="stylesheet" href="style.css" />

    <script src="script.js"></script>

  </head>

  <body>

      <script type="module">

            let a = [4,5,6];

            let b = [1,2,3, ...a, 7,8,9];

            document.write(b); //1,2,3,4,5,6,7,8,9

      </script>

  </body>

</html>

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