Module 4: Constructing Rest Parameters and the Spread Operator

JS Feature works well with functions:

default parameters
rest parameters
spread operator

What are Default Paramters?

Default parameters allow developer provide default values to a parameter in a function

function sayHi(name = 'World') {
    console.log('Hello ' + name);
}
sayHi(); // Hello World
sayHi('John'); // Hello John

function sayHi(message, name = 'World') {
    console.log(message + name);
}
sayHi('Hello'); // Hello World
sayHi('Hi', 'John'); // Hi John

Note: default parameters always appear last in argument list

Constructing Rest Parameters

Rest parameter = a featuer of JS function that deal with multiple argument invocation
Like default parameter, rest parameter come later in function declaration (i.e. after any regular parameters)

e.g. of rest parameters

let setHi = function greet(...names) {
    names.forEach(name => console.log('Hi ' + name));
}
greet('Marry', 'John', 'James');    // Hi Mary
                                    // Hi John
                                    // Hi James

e.g. of rest parameters with other parameters.

let sayHi = function greet(message, ...names){
    console.log(message + ' everyone!');
    names.forEach(name => console.log('Hi ' + name));
}
greet('Welcome', 'Mary', 'John', 'James');  // Welcome everyone!
                                            // Hi Mary
                                            // Hi John
                                            // Hi James

Using the Spread Operator

Reference: How to use the spread operator in javascript

Spread Operator ...:

Opposite of rest operator.
Allow a function take an array as an argument and spread out elements to assign to individual function parameters
- Rest parameters delcared in function parameter
- Spread Operator used in function invokation as argument

function greet(person1, person2) {
    console.log('Hello ' + person1 + ' and ' + person2);
}
let names = ['John', 'Mary'];
greet(...names);

e.g. Spread Operator work with Rest Parameters

function display(char1, char2, char3, char4, ...others) {
    console.log(others);
    console.log(char1, char2, char3, char4);
}
let letters = 'abcdefgh';
display(...leters)