ES6 keywords behaves as variable

[Shaheer Khan]

I have a code that i was expecting will return 'undefined' or any error but it gives output as mentioned.

let i = (x, c) => {
c(x);
};
i(20, (undefined) => {
let j = undefined;
console.log(j); // output 20
});

function y(undefined) {
let a = undefined;
console.log(a); //output 90
}
y(90);
// same result with let,promise,map,filter
How keywords works and change behavior if used as function arguments?