Default Parameters:
If you are familiar with other programming languages like Ruby, Python then default parameters isn’t new to you.
Default parameters are parameters which are given by default while declaring a function. But it’s value can be changed when calling the function.
Example
let Func = (a, b = 10) => {
return a + b;
}
Func(20); // 20 + 10 = 30
In the above example, we are passing only one parameter. The function makes use of the default parameter and executes the function.
Consider another example:
Func(20, 50); // 20 + 50 = 70
In the above example, the function takes two parameters and the second parameter replaces the default parameter.
来源：https://codeburst.io/es6-tutorial-for-beginners-5f3c4e7960be