Type Narrowing Identify Function example

comparisons.ts

type ComparisonFn = (left: number, right: number) => boolean;

// `buildComparison` is a 'Type Narrowing Identity Function'
// It returns the value given to it, but also narrows the types.
// In this case it allows typescript to retain the literal types of the keys
// rather than widening them to 'string'
const buildComparisons = <Comparison extends Record<string, ComparisonFn>>(comparison: Comparison) => comparison;

// Using the Type Narrowing Identity Function:
const comparisons = buildComparisons({
  '=': (left, right) => left === right,
  '≠': (left, right) => left !== right,
  '>': (left, right) => left > right,
  '<': (left, right) => left < right,
  '≥': (left, right) => left >= right,
  '≤': (left, right) => left <= right,
});

type Operator = keyof typeof comparisons;

function compare(left: number, operator: Operator, right: number): void {
  const result = comparisons[operator](left, right);
  console.log(`${left} ${operator} ${right} ? ${result}`)
}

compare(2, '=', 6);
compare(2, '=', 2);
compare(2, '<', 6);
compare(6, '<', 2);
compare(2, '>', 6);
compare(6, '>', 6);
compare(6, '>', 2);
compare(-5, '=', 6);
compare(-5, '=', -5);
compare(0, '>', 6);
compare(0, '>', 0);
compare(0, '≥', 6);
compare(0, '≤', 6);
compare(0, '≠', 0);
compare(2, '≠', 2);
compare(1, '≠', 3);