TypeScript Project: Grade Calculator

What you will build

A grade calculator takes numeric scores, maps each one to a letter, then prints a report with an average. The letter boundaries live in one function so the table and the average never disagree about what 89 means. Scores sit in a number[].

Demo scores are hardcoded. Use Try it in TypeScript and/typescript/try. The Python and C++ try pages will not compile this.

Numeric score to letter

Walk the scale from the top. 90 and up is A, 80 is B, 70 is C, 60 is D, anything below is F. A score outside 0 through 100 still gets a letter here; you can clamp or reject those in practice. The return type isstring.

function letterFrom(score: number): string {
  if (score >= 90) {
    return "A";
  } else if (score >= 80) {
    return "B";
  } else if (score >= 70) {
    return "C";
  } else if (score >= 60) {
    return "D";
  }
  return "F";
}

console.log(91 + " " + letterFrom(91));
console.log(84 + " " + letterFrom(84));
console.log(59 + " " + letterFrom(59));

Output is A, then B, then F. 90 is the first A; 89 is still a B.

Average of an array

Sum the scores, then divide by length. TypeScript number keeps the fraction. An empty array has no average; return 0 and let the caller print a message.

function average(scores: number[]): number {
  if (scores.length === 0) {
    return 0;
  }
  let sum = 0;
  for (const s of scores) {
    sum = sum + s;
  }
  return sum / scores.length;
}

const scores: number[] = [];
scores.push(91);
scores.push(84);
scores.push(76);
console.log(average(scores));

GPA-style points

A GPA-style average maps letters to 4.0, 3.0, 2.0, 1.0, and 0.0, then averages those points. That is not the same as averaging the raw percents. Both numbers are useful; this project prints each.

function letterFrom(score: number): string {
  if (score >= 90) return "A";
  if (score >= 80) return "B";
  if (score >= 70) return "C";
  if (score >= 60) return "D";
  return "F";
}

function pointsFrom(letter: string): number {
  if (letter === "A") return 4.0;
  if (letter === "B") return 3.0;
  if (letter === "C") return 2.0;
  if (letter === "D") return 1.0;
  return 0.0;
}

console.log(pointsFrom(letterFrom(91)));
console.log(pointsFrom(letterFrom(84)));

91 becomes A and 4.0. 84 becomes B and 3.0.

Complete grade report

Print every score with its letter, then the percent average and the GPA-style average, both with two decimal places. Names are optional here; the array is scores only so the arithmetic stays obvious.

function letterFrom(score: number): string {
  if (score >= 90) return "A";
  if (score >= 80) return "B";
  if (score >= 70) return "C";
  if (score >= 60) return "D";
  return "F";
}

function pointsFrom(letter: string): number {
  if (letter === "A") return 4.0;
  if (letter === "B") return 3.0;
  if (letter === "C") return 2.0;
  if (letter === "D") return 1.0;
  return 0.0;
}

const scores: number[] = [];
scores.push(91);
scores.push(84);
scores.push(76);
scores.push(68);
scores.push(95);

console.log("Score  Letter");
let sum = 0;
let pointSum = 0;
for (const s of scores) {
  const letter = letterFrom(s);
  console.log(String(s).padStart(5) + "  " + letter);
  sum = sum + s;
  pointSum = pointSum + pointsFrom(letter);
}

console.log("Average percent: " + (sum / scores.length).toFixed(2));
console.log("GPA-style: " + (pointSum / scores.length).toFixed(2));

Five rows print first. The percent average is 82.80. The GPA-style average uses A, B, C, D, A which is (4+3+2+1+4) / 5 = 2.80.

Common mistakes

• Testing F first with score < 60 and then using if instead of else iffor the rest. A 95 is less than nothing in a broken chain and can fall through. Start from A and go down, or use exclusive ranges.
• Calling letterFrom in the report and a second copy of the scale in the GPA function. One function for the letter; one function from letter to points.
• Dividing by length without checking for an empty array. Guard that case.
• Printing averages without toFixed(2). Then 82.8 and 2.8 look unfinished next to a grade table.

Practice

1. Change the 68 to 88 and confirm the letter becomes B and both averages rise.
2. Add a sixth score of 100 with push and confirm an extra A row appears.
3. Treat scores below 0 or above 100 as invalid instead of a letter, and count how many A grades appear.

Last project: a shopping cart with a receipt and a money total.