Android & Java Engineer
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Read note of TypeScript Basics

Use : to specify type information

function add(a: number, b: number) { return a + b; }

const

const is introducted in ES6. Similar to var, but is block scoped(only exist inside of curly braced block) and can’t be reassigned.

interface

TypeScript allow using interface to define more complex type.

interface Puppy {
  name: string;
  age: number;
};

const realPuppy: Puppy = {
  name: 'Pikachu',
  age: 1
};

const notRealPuppy: Puppy = {
  says: 'meow' // Error: this is clearly not a puppy
}

Arrays

Arrays are defined as Array or Array[]

// define array as Array<Type>
const cats: Array<string> = ['Simba', 'Aslan'];
// Type[] does the same thing.
const cats2: string[] = ['Simba', 'Aslan'];

interface Cat {
  name: string,
  age: number
}

const betterCats: Array<Cat> = [
  {name: 'Simba', age: 22},
  {name: 'Aslan', age: 9999}
];

class

class is used to group method and properties together.

export class Puppy {
  // This is a method.
  bark(){
    // That's how russian dogs talk.
    return 'Gav gav!!';
  }
}

// Now we can instantiate (create) it
var hotdog = new Puppy();
// And use its methods
console.log(hotdog.bark());

constructor

There is a constructor method in class. It’s run when class is instantiated and can take a parameter.

export class Puppy {
  constructor(public name: string){
    // Later we'll have code here
  }
  bark(){
    return 'Gav! my name is ' + this.name;
  }
}

constructor parameter

Constructor parameter marked as public (or private, protected) become class properties.

export

export is used to share information between files.

filter

filter is an Array method and can be used to filter array items.

const numbers = [12,23,62,34,19,40,4,9];

console.log(numbers.filter(function(n: number){
  return n > 30;
}));

// Or use shorthand function notation.
// (Also called arrow function)
console.log(
  numbers.filter(n => n > 30)
);