Create JavaScript Array within For Loop

Question

I want to create the array below with a for loop as its large

var centres = {
 1979: { x: width * 1 / 41, y: height / 2 },
 1980: { x: width * 2 / 41, y: height / 2 },
 1981: { x: width * 3 / 41, y: height / 2 },
 ...
}

and then access it as follows:

function nodeYearPos(d) {
 return yearCenters[d.year].x;
}

I have the following code, but its only setting the year...

 var yearCenters = Array.from(new Array(2020-1919+1), (x, i) => i + 1919);
 for (year = 1919; year <= 2020; year++) {
 coords = getCentres(year); // this returns an object in the form {x : x, y : y}
 yearCenters[year] = coords;
 }

• Daniel A. White: use an object, not an array. arrays are 0-indexed
• Rajneesh: yearCenters = Object.fromEntries(Array.from(new Array(2020-1919+1), (x, i) => [i + 1919,null])); convert the array to object and then set values.

Answer 1

you can do as gorak commented but with the getCenters function

var yearCenters = Object.fromEntries(Array.from(new Array(2020-1919+1), (x, i) => [i + 1919, getCenters(i + 1919)]));

or you can also try

var yearCenters = {};
for (year = 1919; year <= 2020; year++) {
 coords = getCenters(year);
 yearCenters[year] = coords;
}

• Chris: I used the latter suggestion as this is nice and clear with its creation of an empty object ({}) and then populating it.

Answer 2

When you try to fetch by year in yearCenters array (e.g. yearCenters[year]) this won't work since the year is not the index in the array.

I would suggest you first convert the array into a JS object so that indexing on it works with years.

See below snippet -

// Create obejct from array
var yearCenters = Object.fromEntries(Array.from(new Array(2020-1919+1), (x, i) => [i + 1919, null]))
 
// This loop remains same
for (year = 1919; year <= 2020; year++) {
 coords = getCentres(year); // this returns an object in the form {x : x, y : y}
 yearCenters[year] = coords; 
}
// Mock function
function getCentres(year) {
 return {
 x: Math.random() * 100,
 y: Math.random() * 100
 }
}
console.log(yearCenters)