What's that thing in Typescript called?

or: Typescript vocabulary that is impossible to google

• "triple-slash directives" = the thing you put at the top of a js file to load types

• "type narrowing" = Why putting something into a function parameter behaves differently than if you put it into a variable, and then into a function parameter

  • as const can usually fix it

  function fn(a: { b: 'c' }) {}
  fn({b: 'c'}) // uses type narrowing
  
  const var1 = {b: 'c'} // doesn't
  fn(var1) // err
  
  const var2 = {b: 'c'} as const
  fn(var2)

  https://www.typescriptlang.org/play?&q=166#code/GYVwdgxgLglg9mABMMAKAhgLkQb0QI2wHIIjEBfASl3IFgAoBlVHQxEoqxAem8RADOAUwGIoATwAOQxGHQAneXADuMMAHMGDCAgFREANwUBGRAF5cbDuQDcTNEfnHqvREMU8+AEzgiwRKC16HTA9QwUAJnNLYlJyRHRREL07emZHCMogA

• Function that affects parameter's type

  • "type predicates" (used to be called "type guards"): returns boolean
  • "assertion functions": throws

• "tsm": That thing that is better than ts-node

  • Update: there are multiple Node.js TypeScript runtimes now: https://github.com/privatenumber/ts-runtime-comparison