Javascript – Algorithm (3)

How to add a delay inside a loop?

Asynchronous process

const wait = ms =>
    new Promise((resolve, reject) => setTimeout(resolve, ms));

let responseStatus = 0; // 0: Server processing, 1: Succeeded, 2: Failed
while (responseStatus === 0) {
    // If still processing, then request async end point again
    const responseStatus = await ApiCall(checkStatusUrl);
    if (responseStatus === 1) {
        // Done
        console.log('Succeeded to fetch data');
    } else if (responseStatus === 2) {
        // Failed
        console.log('Failed to fetch data');
    }

    await wait(5000); // wait 5s
}

Synchronous process

• for…

var i = 1;                  //  set your counter to 1

function myLoop() {         //  create a loop function
  setTimeout(function() {   //  call a 3s setTimeout when the loop is called
    console.log('hello');   //  your code here
    i++;                    //  increment the counter
    if (i < 10) {           //  if the counter < 10, call the loop function
      myLoop();             //  ..  again which will trigger another 
    }                       //  ..  setTimeout()
  }, 3000)
}

myLoop();                   //  start the loop

for (let i=0; i<10; i++) { 
   task(i); 
} 
  
function task(i) { 
  setTimeout(function() { 
      // Add tasks to do 
  }, 2000 * i); 
} 

• while…

let i = 0; 
while (i < 10) { 
  task(i); 
   i++; 
} 
function task(i) { 
  setTimeout(function() { 
      // Add tasks to do 
  }, 2000 * i); 
}

How to sort an array effeciently?

Use https://github.com/jherax/array-sort-by

// Input
let rivers = ['Nile', 'Amazon', 'Congo', 'Mississippi', 'Rio-Grande'];

// Output
[ 'Nile', 'Congo', 'Amazon', 'Rio-Grande', 'Mississippi' ]

// temporary array holds objects with position
// and length of element
var lengths = rivers.map(function (e, i) {
    return {index: i, value: e.length };
});

// sorting the lengths array containing the lengths of
// river names
lengths.sort(function (a, b) {
    return +(a.value > b.value) || +(a.value === b.value) - 1;
});

// copy element back to the array
var sortedRivers = lengths.map(function (e) {
    return rivers[e.index];
});

How to count number of group from an array?

// How many group having 20 elements?
const GROUP_LENGTH = 20
const arr = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]

const arrLength = arr.length;
const remainder = arrLength % GROUP_LENGTH;  // Get remain
const lessTotal = Math.floor(arrLength / GROUP_LENGTH);  // Get division without remain

const count = remainder > 0 ? lessTotal + 1 : lessTotal;  // Count group number

for (let i = 0; i < count; i += 1) {
  // Get array of 20 items
  const start = i * GROUP_LENGTH;
  let twentiesItems = [];
  if (i !== count - 1) {
    twentiesItems = arr.slice(start, GROUP_LENGTH); // Get 20 items
  } else {
    twentiesItems = arr.slice(start); // Get the rest of items
  }

  // Do something with array of 20 items twentiesItems
  ...
}