Tag Archives: cheat sheet

Simple use of Java Script spread operator

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Converting array into object

// The array
var numbers = [1,2,3];


var obj = {...numbers }

console.log(obj)
// result: {0:1, 1:2, 2:3}

 

Expanding object with new parameter

// The array
var person = {
  name: 'Jhon'
}

var obj = {...person, age: 23 }

console.log(obj)
// expected person to be: {name: 'John', age:23 }

 

Getting sum of an array

function sum(x, y, z) {
  return x + y + z;
}

const numbers = [1, 2, 3];

console.log(sum(...numbers));
// resunt: 6

 

Understanding Java Script Map, Filter and Reduce

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The array

// The array
var people = [
  { name: 'john', age: 24, weight: 84 },
  { name: 'tim', age: 34, weight: 76 },
  { name: 'Jack', age: 26, weight: 65 },
];

 

MAP

// map - use it if I want to do the same operation for each element in the array
// and return the same amount of items in the array.
// array.map(
// current item of the array,
// curent index,
// the entire array
// );
var result = people.map((person, index, persons) => {
  return `${person.name}: ${person.age}`;
});
console.log('map', result);

 

FILTER

// filter - return only items that match certain criteria.
// array.filter(
// current item of the array,
// curent index,
// the entire array
// )

var result = people.filter((person, index, persons) => {
  return person.age < 30;
});
console.log('filter', result);

 

REDUCE

// reduce - return something completely new after iteration through each element
// in the array
// array.reduce(
// current value of the end value,
// current item,
// curent index,
// the entire array
// )
var initialValue = 10;
var combined_weight = people.reduce((weight, person, index, persons) => {
  return weight += person.weight;
}, initialValue);
console.log('reduce', combined_weight);

Another example of reduce, that will convert the array of objects, to object of objects, where person.name is going to be the key

var reducer = (accumolator, person) => {
  accumolator[person.name] = person;
  return accumolator;
}

var initialValue = {};

// reduce takes reducer function as first parameter, and initial value
var result = people.reduce(reducer, initialValue);

console.log('reduce', result);