In Software development
var arr = ['a', 'b', 'c', 'd', 'e'];
for (var i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}

The example array

var people = [{
  id: 1,  
  name: 'Alice',
  age: 16
},
{
  id: 2,
  name: 'Bob',
  age: 19
},
{
  id: 3,
  name: 'Carol',
  age: 15
},
{
  id: 4,
  name: 'David',
  age: 21
}];

Sum an array

var sum = 0;for (var i = 0; i < people.length; i++) {
  sum += people[i].age;
}
const sum = people.reduce(function(acc, person){
  return acc + person.age;
}, 0)
const sum = people.reduce((acc, person) => (acc + person.age), 0);

Interlude: a note about arrow functions

Filter an array

var adults = [];for (var i = 0; i < people.length; i++) {
  if (people[i].age > 16) {
    adults.push(people[i]);
  }
}
const adults = people.filter(person => person.age > 16);

Find the element with max property

var oldest = {age: 0};for (var i = 0; i < people.length; i++) {
  if (people[i].age > oldest.age) {
    oldest = people[i];
  }
}
const oldest = people.reduce((acc, person) => 
  person.age > acc.age ? person : acc
);

Extract a property from an array of objects

var names = [];for (var i = 0; i < people.length; i++) {
  names.push(people[i].name);
}
const names = people.map(val => val.name);

Index an array of objects

var iPeople = {};for (var i = 0; i < people.length; i++) {
  iPeople[people[i].id] = people[i];
}
const iPeople = people.reduce((acc, person) => {
  acc[person.id] = person;
  return acc;
}, {});

Find an element in array

var details = null;for (var i = 0; i < people.length; i++) {
  if (people[i].name === 'Bob'){
    details = people[i];
    break;
  }
}
const details = people.find(val => val.name === 'Bob');

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