learning_javascript_typescript_compiler.md

path	title
/learnings/javascript_typescript_compiler	Learnings: Javascript: Typescript: Compiler

Table Of Contents

• Typescript Compiler
  • compiler definition talk about typescript
    • Open vs closed types
  • Compiler Behavior and you
    • Runtime constructs vs Compile time constructs
    • Excess property checking
  • Places where the compiler can help you
    • reducing the amount of types you actually have to write
      • potential way to lint places where you don't have to write types
  • Wrting code to give more hints to the compiler
    • any vs unknown
    • declare more variables as const
    • Always annotate return type

Typescript Compiler

compiler definition talk about typescript

Open vs closed types

• [BOOKNOTES]:

“sealed” or “precise” type, and it cannot be expressed in TypeScript’s type system. Like it or not, your types are “open.”

• From: Effective Typescript

Compiler Behavior and you

Runtime constructs vs Compile time constructs

• [BOOKNOTES]:

Some constructs introduce both a type (which is not available at runtime) and a value (which is). The class keyword is one of these.

• From: Effective Typescript

Excess property checking

• [BOOKNOTES]:

When you assign an object literal to a variable or pass it as an argument to a function, it undergoes excess property checking.

• From: Effective Typescript

• [BOOKNOTES]:

When you assign an object literal to a variable with a declared type, TypeScript makes sure it has the properties of that type and no others:

• From: Effective Typescript
• [TODO]: wite me

Places where the compiler can help you

• [BOOKQUOTE]:

There are some situations where you can leave the type annotations off of function parameters, too. When there’s a default value, for example:

• From: Effective Typescript

reducing the amount of types you actually have to write

potential way to lint places where you don't have to write types

check out the eslint rule no-inferrable-types

Wrting code to give more hints to the compiler

any vs unknown

• [BOOKQUOTE]:

unknown is appropriate whenever you know that there will be a value but you don’t know its type

• from: Effective Typescript

declare more variables as const

• [BOOKQUOTE]:

Because x cannot be reassigned, TypeScript is able to infer a narrower type without risk of inadvertently flagging errors on subsequent assignments.

• From: Effective Typescript

Always annotate return type

• [BOOKQUOTE]:

When you annotate the return type, it keeps implementation errors from manifesting as errors in user code.

• From: Effective Typescript

• [BOOKQUOTE]:

This is a good reason to consider including explicit return type annotations, even when the return type can be inferred. It prevents an any type from “escaping.”

• from: Effective Typescript