type system

Wilson Mendes

@willmendesneto

Google Developer Expert Web Technologies

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Sometimes, life without types can be hard...

Code confidently

Type inference

Catch errors easily

Why type check?

Easy to adopt, easy to leave

Easy to maintain

No Runtime exceptions as goal

Flow?

What about Typescript?

EX: Non-Nullable types

Typescript

parametric types compatible

* if the type to assign from has a more special type parameter

Flow

Expressive

Easy to add, easy to remove

// Typescript


function foo(num: number) {
    if (num > 10) {
      return 'cool';
    }
}

// It works!
const result: string = foo(100);
console.log(result.toString());

// It doesn't work
console.log(foo(1).toString());

// error at runtime
"Cannot read property 'toString' of undefined"

// Flow


function foo(num: number) {
    if (num > 10) {
        return 'cool';
    }
}


// error: call of method `toString`.
console.log(foo(100).toString());


// Error:
"Method cannot be called on possibly null value"

Typescript is a compiler

Flow is a checker

Flow x Typescript

* just a flame, relax

// Flow usage


// Initialize your project

flow init

// Start background process

flow

// @flow
// test.js

function foo(x: ?number): string {
  if (x) {
    return x;
  }
  return "default string";
}

test.js:5
  5:     return x;
                ^ 
number. This type is incompatible with the 
expected return type of

  3: function foo(x: ?number): string {
                               ^^^^^^ string