Rest parameter
When creating a function, you can add the symbol ‘…’ to the left of the parameter.
Then something strange happens.
function fn2(...parameter) {
console.log(parameter);
}
fn2(1, 2, 3, 4, 5, 6, 7);
If you run the above code, it will output a variable called parameters.
A variable called parameters contains all parameters in a [] array.
This is a rest parameter that can be used in the ES6 environment.
Add ... to the left of the desired parameter
It means “parameters enclosed in [] curly braces around all the parameters that come in this place”.
So if you print it out, [1,2,3,4,5,6,7] comes out.
Rest parameters are all parameters (1,2,3,4,5,6,7) wrapped in braces.
If ... is used elsewhere
If you said to wrap the parameter in its place in []
What if I put it somewhere else?
function fn2(a, b, ...parameters) {
console.log(parameters);
}
fn2(1, 2, 3, 4, 5, 6, 7);
If you run the above code, it will output [3,4,5,6,7].
The first two parameters are written as a, b.
All parameters after a,b are wrapped in curly braces to become an array called parameters
When there are many types of parameters, it is easier to handle and simpler than the arguments syntax, so it is often used.
Notes before writing
Rest(remaining) As the parameter means, it can be used only in the rest part.
So, if there are always multiple parameters, rest should always be put as the last parameter.
function fn2(a, ...parameters, b){
console.log(parameters)
}
You’re getting an error if you use it like this.
function fn2(a, ...parameters, ...parameters2){
console.log(parameters)
}
this also doesn’t work. You cannot use more than 2.
Simple exercises
I want to create a function that outputs all the parameters one by one to the console window.
What should I do?
It’s easy, so let me give you the answer.
function fn(a, b, c) {
console.log(a);
console.log(b);
console.log(c);
}
fn(1, 2, 3);
However, if 4 or 5 parameters were entered incorrectly, a stupid function that did not run properly was completed.
How can I create a function that performs the same function without limiting the number of parameters?
A.
function fn(...rest){
console.log(rest[0]);
console.log(rest[1]);
console.log(rest[2]);
(...)
}
fn(1,2,3);
can’t you do it like this But since I don’t know how many parameters
The console.log() part can be made simpler by using a loop.
function fn(...rest) {
for (var i = 0; i < rest.length; i++) {
console.log(rest[i]);
}
}
fn(1, 2, 3, 4, 5, 6, 7, 8);
Because rest is a variable that puts all parameters in [] and makes it like an array.
You can turn the loop like that.
Then, no matter how many parameters you put in the function, you can continue to output to the console window.
