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How to get previous month name on given some month (in words not numerical value)? [duplicate]

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How to get previous month (DECEMBER) name
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I want a function that gives the full name of prev month for any given month[not just current month]
Ex: prevMonth(August) = July; prevMonth(January)= December.
I'm new to js and can't figure out how to use this array to get the result:
monthsarray: ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]

Are expecting a simple solution like this?
var monthsarray = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]
function prevMonth(curMonth) {
var curMonthIndex = monthsarray.indexOf(curMonth);
var prevMonthIndex;
if (curMonthIndex == 0) prevMonthIndex = monthsarray.length - 1;
else if (curMonthIndex == monthsarray.length - 1) prevMonthIndex = 0;
else prevMonthIndex = curMonthIndex - 1;
return monthsarray[prevMonthIndex];
}
console.log(prevMonth("August"));
console.log(prevMonth("December"));

let monthsarray1= ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"] ;
let m="May";
console.log(pvsMonth(m))
function pvsMonth(m){
let ln=monthsarray1.length
let idx=monthsarray1.indexOf(m)
if(idx==0){
return monthsarray1[ln-1]
}else{
return monthsarray1[idx-1]
}
}

function prevMonth(month) {
const index = monthsarray.indexOf(month);
if (index > 0) {
return monthsarray[index - 1];
}
if (index === 0) {
return monthsarray[monthsarray.length - 1];
}
throw new Error('Invalid Month given');
}

Results from loop don't work correctly

I want to loop through an array.
Any index which has a length larger than 3 I want to abbreviate.
I want to place the new abbreviated months into a new array.
I want to test that it works by writing the results to the console.
I can get the code to run, however the results don't come out the way I'd hope. From my understanding the loop runs true, running the if statement, which runs true, running the code block. After that the loop should iterate and continue as long as i < months.length, but it doesn't.
var months = ["January", "Febuary", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];
var monthsAbbrev = [];
for (var i = 0; i < months.length; i++) {
if (months[i].length > 3) {
monthsAbbrev = months[i].slice(0, 3);
}
}
console.log(monthsAbbrev);

You can do it simply with Array.prototype.map()
var months = ["January", "Febuary"]; //sample data for better understanding
var monthsAbbrev = months.map(v => v.substr(0,3));
console.log(monthsAbbrev); //["Jan", "Feb"]
By the way you are not pushing anything into your target array in your code, that is the problem.

Beside the pushing, you can omit the check for length > 3, because slice is already doing it and that prevent to miss some month, like 'May'.
var months = ["January", "Febuary", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];
var monthsAbbrev = [];
for (var i = 0; i < months.length; i++) {
monthsAbbrev.push(months[i].slice(0, 3));
}
console.log(monthsAbbrev);

You need to add the months to your monthsAbbrev array. One way you can do this is by using the .push() function:
var months = ["January", "Febuary", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];
var monthsAbbrev = [];
for (var i = 0; i < months.length; i++) {
if (months[i].length > 3) {
monthsAbbrev.push(months[i].slice(0, 3));
}
}
console.log(monthsAbbrev);

You just had to push to the array formed. Rest you already had it.
var months = ["January", "Febuary", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];
var monthsAbbrev = [];
for (var i = 0; i < months.length; i++) {
if (months[i].length > 3) {
monthsAbbrev.push(months[i].slice(0, 3));
}
}
console.log(monthsAbbrev);

Assign value in for loop

I am trying to get the actual month from a loop. When I step through it in developer tools and month[i] == 4 it doesn't assign actualMonth to checkMonth
Do I have to assign getMonth to month[] and then try and query the value?
var showCurrentMonth = function() {
var getMonth = new Date().getMonth();
var month = ["january", "february", "march", "april", "may", "june", "july", "august", "september", "october", "november", "december"];
var actualMonth = "";
for (var i = 0; i < month.length; i++) {
var checkMonth = month[i];
console.log(month[i]);
if (getMonth == month[i]) {
actualMonth = checkMonth;
}
}
console.log(actualMonth);
}
window.addEventListener('DOMContentLoaded', showCurrentMonth, false);

Too simple?
var showCurrentMonth = function() {
var getMonth = new Date().getMonth();
var month = ["january", "february", "march", "april", "may", "june", "july", "august", "september", "october", "november", "december"];
var actualMonth = month[getMonth];
console.log(actualMonth);
}
window.addEventListener('DOMContentLoaded', showCurrentMonth, false);

Change your if to if (month[getMonth] == month[i]) {
Do it like this :
var showCurrentMonth = function() {
var getMonth = new Date().getMonth();
var month = ["january", "february", "march", "april", "may", "june", "july", "august", "september", "october", "november", "december"];
var actualMonth = "";
for (var i = 0; i < month.length; i++) {
var checkMonth = month[i];
console.log(month[i]);
if (month[getMonth] == month[i]) { //Compare get month like this
actualMonth = checkMonth;
}
}
console.log(actualMonth);
}
window.addEventListener('DOMContentLoaded', showCurrentMonth, false);

Your problem is that you are comparing an integer and string:
var getMonth = new Date().getMonth(); // This return number from 0 to 11
The code Date().getMonth() returns an integer, and your month list has strings on it
Your code should be:
var showCurrentMonth = function() {
var getMonth = new Date().getMonth();
var month = ["january", "february", "march", "april", "may", "june", "july", "august", "september", "october", "november", "december"];
var actualMonth = month[getMonth];
console.log(actualMonth);
}
window.addEventListener('DOMContentLoaded', showCurrentMonth, false);
To get the actual month you only need to access the month list with getMonth as index

slightly Modification required
var showCurrentMonth = function() {
var getMonth = new Date().getMonth();
console.log(getMonth);
var month = ["january", "february", "march", "april", "may", "june", "july", "august", "september", "october", "november", "december"];
var actualMonth = "";
for (var i = 0; i < month.length; i++) {
var checkMonth = month[i];
if (getMonth == i) {
actualMonth = checkMonth;
}
}
console.log(actualMonth);
};
window.addEventListener('DOMContentLoaded', showCurrentMonth, false);

Looping through an array to div?

I'm trying to add a p element containing a single month in this string to a div with the id of "write". How do I get this loop to run through the list of strings and add them to the div?
var months = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October","November", "December"];
var element = document.getElementById("write").innerHTML = months;
for (var i = 0; i <= months; i++) {
var para = document.createElement("p");
var text = document.createTextNode(i);
}
I don't understand why it isn't working. Thanks!

It looks like you're totally misunderstanding how to use arrays, you need to go back to your tutorials and study this.
You need to use months.length as the limit of the array, not just months. You should use <, not <= in the test, because otherwise you'll go past the end of the array. And in the text node you should put months[i], not just i. Then you need to make the text node a child of the paragraph. You can't set the innerHTML of an element to an array; you should be adding the paragraphs as children of the element.
var months = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];
var element = document.getElementById("write");
for (var i = 0; i < months.length; i++) {
var para = document.createElement("p");
var text = document.createTextNode(months[i]);
para.appendChild(text);
element.appendChild(para);
}
<div id="write"></div>

How to check if a string is a substring of any element in an array

I have an array
var months = ["January", "February", "March", "April", \
"May", "June", "July", "August", "September", "October", \
"November", "December"];
I have strings like "Nov", "October", "Jun", "June", "Sept", "Sep" etc. The point is, the string can be a substring of one of the months.
What is the best way to compare the string to be a substring of the array elements? How do I find out the index of the month?
I am looking at javascript/jQuery.
I know I can just loop through the array checking against each element using search and break when found. I want something better.

var month_index = function(target) {
target = target.toLocaleLowerCase();
return jQuery.inArray(true, jQuery.map(months, function(s) {
return s.toLocaleLowerCase().indexOf(target) > -1;
}))
};
var index_of_october = month_index("oct");

var substr = 'nov', months = ["december", "november", "feb"];
var index = months.indexOf( months.filter(function(v){ return v.indexOf(substr) >= 0;})[0]);
alert(index)
http://jsfiddle.net/KeMe9/

Put your array items in lower case
var txt = "Sep";
var found = false;
txt = txt.toLowerCase();
for (var i = 0; i < 12; i++) {
if (months[i].indexOf(txt) > -1) {
found = true;
break;
}
}