Features of ES6 Part 4: Rest Parameters

A rest parameter allows a function to work with an unknown or variable number of arguments. A function’s rest parameter always appears at the end of the function’s argument list and uses a triple dot prefix (...), as shown below.

let doWork = function(name, ...numbers){
    let result = 0;
    
    numbers.forEach(function(n){
        result += n;
    });

    return result;
};

A caller invoking doWork can pass zero or more parameters at the rest parameter position. You can say the numbers argument will take “the rest” of the parameters a caller passes, and numbers will hold the parameter values in an array. In the following example, numbers will reference an array with the values 1, 2, 3.

let result = doWork("Scott", 1, 2, 3);
expect(result).toBe(6);

In the case where a caller passes no parameters in the rest parameter position, the rest parameter will be an empty array.

let doWork = function(...numbers){
    return numbers;
};

let result = doWork();
expect(result.length).toBe(0);

The Impact of Rest Parameters

Before ES6, we could allow callers to pass a variable number of arguments to a function by using the implicit arguments variable inside of the function. The arguments variable contains all the parameters to a function in an array-like object, but arguments is not an array, which creates confusion. It is also difficult to spot if a function is using arguments without reading through the code or documentation for the function.

ES6 rest parameters will avoid the confusion by always giving us a true array, and by using a dedicated syntax that makes rest parameters easy to spot when reading the function signature.

Want more? Watch JavaScript Fundamentals for ES6 on Pluralsight!