const daysFromNow = (date) => Math.round((date-Date.now())/(1000*3600*24))
console.log(daysFromNow(new Date('07/04/2020')));
//result is -534 Given the above, why does it work? I've read that .now() gives back the ms since 1970. Does the passed in date hold a value of the ms since 1970 as well?
Does the passed in date hold a value of the ms since 1970 as well?
The Date object represents a specific moment on earth, so yes you can retrieve that the milliseconds since 1970 in milliseconds.
A Date object has a valueOf function that returns the primitive value of the Date:
The number of milliseconds between 1 January 1970 00:00:00 UTC and the given date.
So (new Date()).valueOf() and Date.now() are equivalent:
console.log((new Date()).valueOf())
console.log(Date.now())And the valueOf is called for conversion in the case of date - Date.now() because date is a Date object and Date.now() returns a number.
Here an example of a custom object with a valueOf function:
let test = {
valueOf() {
return 5;
}
}
console.log(test - 1);That's work because of valueOf property. That property return a number, implemented in all objects in js. You can also owerride it in your objects.
Here an example of using for creating add chain js function.
function add(x) {
const fn = (y) => add(x + y);
fn.valueOf = () => x;
return fn;
}
const addTen = add(5)(5);
console.log(addTen + 1) // 11