JavaScript Create an array from a for loop

Question

I want to calculate the age of some persons(birthyears are stored inside an array) using a function and then creating another array with the ages from the for loop but i keep getting an empty array. Can anyone help?

 const years = [1991, 1994, 2008, 2020];
 const calcAge = function (birthYear) {
 return 2021 - birthYear;
};
 const age = [];
 for (let i = 0; i > years.length; i++) {
 calcAge(i);
 age.push(i);
}
 console.log(age);

• Tushar Shahi: press.rebus.community/programmingfundamentals/chapter/for-loop Check this out.
• gen_Eric: i > years.length will always be false.
• Tushar Shahi: i starts with 0 and the condition is that is should be greater then 4(arr length). This is never true and hence empty array.
• Tushar Shahi: You also might want to change the code inside your loop but once you fix the for loop conditions, doing console.log() will let you know what the issue is.
• Nur: This main problem is complexity, Just keep it simple... Use years.map(birthYear => 2021 - birthYear)

Answer 1

You're simply using > instead of <:

const years = [1991, 1994, 2008, 2020];
const calcAge = function (birthYear) {
 return 2021 - birthYear;
};
const age = [];
for (let i = 0; i < years.length; i++) {
 age.push(calcAge(years[i]));
}
console.log(age);

The loop should run until i becomes equal to years.length. If i starts off as 0, the comparison i > years.length will be false (since 0 > 4 === false), and the loop will immediately terminate.

I've made one more improvement for you; I switched:

calcAge(i);
age.push(i);

to:

age.push(calcAge(years[i]));

The former has a number of issues; you pass i, the index, instead of years[i], the actual age. And you perform the calcAge computation but never actually store the answer. The latter fixes these issues.

As suggested by Nur, you could simplify your code using map:

const years = [ 1991, 1994, 2008, 2020 ];
const ages = years.map(year => 2021 - year);
console.log(ages);

• Nur: use for..of loop
• Gershom Maes: for..of or Array.prototype.map would be cleaner solutions, but I want to address the actual question asked by the OP.

Answer 2

your for loop condition isn't right. it should be switched from i > years.length to i < years.length. You also need to push the return value of your age calculation to the ages array

const years = [1991, 1994, 2008, 2020];
 const calcAge = function (birthYear) {
 return 2021 - birthYear;
};
 const ages = [];
 for (let i = 0; i < years.length; i++) {
 const age = calcAge(years[i]);
 ages.push(age);
}
 console.log(ages);