index.ts

// // console.log("Running TypeScripts");
// // var schnema: number = 1;
// // schnema = 3;

// // let schnema2: string = "Ascon";
// // schnema2 ='ray';


//EXAMPLE 1
// /*a promise that expects a reult,it takes a call back function and the function requires a parameter
// //eg fetch() uses a promise.*/
// //because promise return something, we need to pass it into a variable, in this case the promise is passed into let res.
let res = new Promise(function (resolve, reject) { // example of a promise that does not begin with a function; you cannot pass in a parameter/value into it.
  //the let res is a return value
  let username = "Sofy"; // here is where we declare the value inside the promise
  if (username.length == 4) {
    resolve("positive");
  } else {
    reject("negative");
  }
});
 /*the message and err passed in as parameters inside the function in the console.log allows us to be able
to be able to access the parameter "positive" passed into the resolve()function AND if  the
result is negative, the parameter err passed in as a call back function into catch()functiom allows us  to be able
to access the negative parameter passed into the reject() function.*/
res.then(function (message) {
    let output = "user is" + " " + message;
    console.log(output);
  }) .catch(function (err) {
    let output = "Ans";
    console.log(output);
  });

//EXAMPLE 2
let ping = new Promise(function (res, rej) {
  if (true) {
    res("good");
  } else {
    rej("bad");
  }
});

ping.then(function(mess) {
    console.log(sum());
  }).catch();

function sum(): number { //this is how to write typescript: it means the function sum() returns number.
  return 2 + 2;
}

//in typescript onces you declare a return, you must specify the return value

//in the first funciton, you can pass a value/parameter inside the first, while the second can only return a promise, one cannot interact or pass in a value.

//EXAMPLE 3: A function that returns a promise, you can pass a parameter inside the function
function res1(dt: string): Promise<string> { // means res1()returns a promise and the promise returns a string; while parameter dttakes a string.
  return new Promise(function (resolve, reject) {
    if (dt.length == 4) {
      resolve("valid");
    } else {
      reject("invalid");
    }
  });
}

res1("oluchi")// here is where value that was passed into the function
  .then(function (message) {
    console.log(message);
  })
  .catch(function (err) {
    console.error(err);
  });

//its best to get a postive response before checking for invalid responses so that an async function will work properly with await

//EXAMPLE 4
function res2(dt: string): Promise<string> { // example of a function that returns a promise
  return new Promise(function (resolve, reject) {
    if (dt.length == 4) {
      setTimeout(function () {
        resolve("valid");
      }, 2000);
    } else {
      reject("invalid");
    }
  });
}

async function simple_async() {
  try {
    const rs = await res2("Dami");
    console.log(rs);
    const rs1 = await res2("Oluchi");
    console.log(rs1);
  } catch (err) {
    console.log(err);
  }
}
simple_async();
console.log("run first");