How to loop through an array containing objects an

Question

I want to cycle through the objects contained in an array and change the properties of each one. If I do this:

for (var j = 0; j < myArray.length; j++){

console.log(myArray[j]);

}

The console should bring up every object in the array, right? But in fact it only displays the first object. if I console log the array outside of the loop, all the objects appear so there's definitely more in there.

Anyway, here's the next problem. How do I access, for example Object1.x in the array, using the loop?

for (var j = 0; j < myArray.length; j++){

console.log(myArray[j.x]);

}

This returns "undefined." Again the console log outside the loop tells me that the objects all have values for "x". How do I access these properties in the loop?

I was recommended elsewhere to use separate arrays for each of the properties, but I want to make sure I've exhausted this avenue first.

Thank you!

Answer 1

This would work. Looping thorough array(yourArray) . Then loop through direct properties of each object (eachObj) .

yourArray.forEach( function (eachObj){
    for (var key in eachObj) {
        if (eachObj.hasOwnProperty(key)){
           console.log(key,eachObj[key]);
        }
    }
});

Answer 2

myArray[j.x] is logically incorrect.

Use (myArray[j].x); instead

for (var j = 0; j < myArray.length; j++){
  console.log(myArray[j].x);
}

Answer 3

In ECMAScript 2015 aka ES6, you can use a for..of loop to loop over an array of objects.

for (let item of items) {
    console.log(item); // Will display contents of the object inside the array
}

At the time of posting this answer, support is pretty non-existent for Internet Explorer, but through the use of a transpiler like Traceur or Babel, you can use new Javascript features like this without really having to worry about what browsers support what.

Answer 4

Some use cases of looping through an array in the functional programming way in JavaScript:

1. Just loop through an array

const myArray = [{x:100}, {x:200}, {x:300}];

myArray.forEach((element, index, array) => {
    console.log(element.x); // 100, 200, 300
    console.log(index); // 0, 1, 2
    console.log(array); // same myArray object 3 times
});

Note: Array.prototype.forEach() is not a functional way strictly speaking, as the function it takes as the input parameter is not supposed to return a value, which thus cannot be regarded as a pure function.

2. Check if any of the elements in an array pass a test

const people = [
    {name: 'John', age: 23}, 
    {name: 'Andrew', age: 3}, 
    {name: 'Peter', age: 8}, 
    {name: 'Hanna', age: 14}, 
    {name: 'Adam', age: 37}];

const anyAdult = people.some(person => person.age >= 18);
console.log(anyAdult); // true

3. Transform to a new array

const myArray = [{x:100}, {x:200}, {x:300}];

const newArray= myArray.map(element => element.x);
console.log(newArray); // [100, 200, 300]

Note: The map() method creates a new array with the results of calling a provided function on every element in the calling array.

4. Sum up a particular property, and calculate its average

const myArray = [{x:100}, {x:200}, {x:300}];

const sum = myArray.map(element => element.x).reduce((a, b) => a + b, 0);
console.log(sum); // 600 = 0 + 100 + 200 + 300

const average = sum / myArray.length;
console.log(average); // 200

5. Create a new array based on the original but without modifying it

const myArray = [{x:100}, {x:200}, {x:300}];

const newArray= myArray.map(element => {
    return {
        ...element,
        x: element.x * 2
    };
});

console.log(myArray); // [100, 200, 300]
console.log(newArray); // [200, 400, 600]

6. Count the number of each category

const people = [
    {name: 'John', group: 'A'}, 
    {name: 'Andrew', group: 'C'}, 
    {name: 'Peter', group: 'A'}, 
    {name: 'James', group: 'B'}, 
    {name: 'Hanna', group: 'A'}, 
    {name: 'Adam', group: 'B'}];

const groupInfo = people.reduce((groups, person) => {
    const {A = 0, B = 0, C = 0} = groups;
    if (person.group === 'A') {
        return {...groups, A: A + 1};
    } else if (person.group === 'B') {
        return {...groups, B: B + 1};
    } else {
        return {...groups, C: C + 1};
    }
}, {});

console.log(groupInfo); // {A: 3, C: 1, B: 2}

7. Retrieve a subset of an array based on particular criteria

const myArray = [{x:100}, {x:200}, {x:300}];

const newArray = myArray.filter(element => element.x > 250);
console.log(newArray); // [{x:300}] 

Note: The filter() method creates a new array with all elements that pass the test implemented by the provided function.

8. Sort an array

const people = [
  { name: "John", age: 21 },
  { name: "Peter", age: 31 },
  { name: "Andrew", age: 29 },
  { name: "Thomas", age: 25 }
];

let sortByAge = people.sort(function (p1, p2) {
  return p1.age - p2.age;
});

console.log(sortByAge);

9. Find an element in an array

const people = [ {name: "john", age:23},
                {name: "john", age:43},
                {name: "jim", age:101},
                {name: "bob", age:67} ];

const john = people.find(person => person.name === 'john');
console.log(john);

The Array.prototype.find() method returns the value of the first element in the array that satisfies the provided testing function.

References

• Array.prototype.some()
• Array.prototype.forEach()
• Array.prototype.map()
• Array.prototype.filter()
• Array.prototype.sort()
• Spread syntax
• Array.prototype.find()

Answer 5

Array object iteration, using jQuery, (use the second parameter to print the string).

$.each(array, function(index, item) {
       console.log(index, item);
});

Answer 6

Use forEach its a built-in array function. Array.forEach():

yourArray.forEach(function (arrayItem) {
    var x = arrayItem.prop1 + 2;
    console.log(x);
});