Meaning of === with function call

Question

I had been going through the ES6 assuming that it would be easy to switch to EcmaScript 2017.

While going through, I got confused about this code

function f (x, y = 7, z = 42) {
    return x + y + z
}
f(1) === 50

Which has ES5 equivalent

function f (x, y, z) {
    if (y === undefined)
        y = 7;
    if (z === undefined)
        z = 42;
    return x + y + z;
};
f(1) === 50;

I did understand the default parameter thing from it.

But what does f(1)===50 mean in both the codes? Whats the use of it?

Here is another example

function f (x, y, ...a) {
    return (x + y) * a.length
}
f(1, 2, "hello", true, 7) === 9

What does f(1, 2, "hello", true, 7) === 9 mean?

I understand that === for comparison between the LHS and RHS of the operator including the type of both and not just value.

But why have it been used like that??

Kindly explain its usage.

This is the link from where I got this. http://es6-features.org/#RestParameter

Answer 1

This is a strict comparison test whether function f(x,y,z), when called with an x parameter value of 1 returns the value 50. This would be true when the default parameter values added to the value of x are 7 and 42.

These function calls and comparisons are purely to provide usage examples and possibly test examples for the functions they call.

The code

function f (x, y, ...a) {
    return (x + y) * a.length
}
f(1, 2, "hello", true, 7) === 9

is an example of extended parameter handling. the ...a variables length property equates to 3 thus the test is confirming the number of parameters passed to the function after x and y.