Type Predicates

Type predicates are functions that return a boolean value. They are used to narrow the type of a variable. Type predicates are used in type guards.

function isString(value: unknown): value is string {
  return typeof value === 'string';

function example(x: unknown) {
  if (isString(x)) {
    // We can now call any 'string' method on 'x'.
  } else {

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