formats letter grades using [threshold, character] pairs

formatLetterGrade.ts

#!/usr/bin/env ts-node 

const thresholds = [
  90,
  80,
  70,
  60,
  0,
];

const letters = [
  'A',
  'B',
  'C',
  'D',
  'F',
];

const signThresholds = [
  6,
  3,
  0,
];

const signs = [
  '+',
  '',
  '-',
];

const formatLetterGrade = (val: number) => {

  const letter = 
    letters[thresholds.findIndex((threshold) => val >= threshold)];
  let sign
    = signs[val === 100 ? 0 : signThresholds.findIndex(threshold => val % 10 >= threshold)];

  return letter + sign;
}

console.log('A+', formatLetterGrade(100));
console.log('A+', formatLetterGrade(96));
console.log( 'B+', formatLetterGrade(89));
console.log( 'C-', formatLetterGrade(70));
console.log( 'C ', formatLetterGrade(73));
console.log( 'C ', formatLetterGrade(75));
console.log( 'D+', formatLetterGrade(69));
console.log( 'F-', formatLetterGrade(50));