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What does colon ':' in API request mean?

i am using firebase for my project ,the documentation gives me the endpoint for signing in users as:
https://identitytoolkit.googleapis.com/v1/accounts:signInWithPassword?key=[API_KEY]
i want to know what does the colon : mean, for example the word key after the question mark shows its a parameter likewise what does the notion accounts:signInWithPassword mean.The reason:I have an axios instance with config:
axios.create(
{
baseURL:"https://identitytoolkit.googleapis.com/v1",
params:{
apiKey:"somekey"
}
})
now since the baseUrl shown above remains same for firebase signing in with email and password or signing up with email and password. I want to dynamically embed accounts:signInWithPassword and accounts:signUp for respective requests and i am not sure if specifying accounts:respectiveUsecase in params object would work.

A colon doesn't have any special meaning in an URL path. It's just a convention those APIs tend to use in their paths.
There are a handful of metacharacters that do:
question marks (?) and hashes (#) delimit the query or search parts
% is used for escaping characters (e.g. %0A)
+ is sometimes an encoding for a space instead of %20.
& generally separates query parameters (e.g. foo=bar&baz), though this is not a part of the standard. Some server software could expect e.g. semicolon-separated parameters.
As #deceze pointed out, colons do have a special meaning in the host part, e.g. https://user:pass#host/path:where:colons:do:not:matter.

It is a dynamic value (like a parameter where you in pass in a value directly)
:nounId: The colon (:) before the word indicates that we don't mean the literal string "nounId" as part of the endpoint, but rather that we are expecting some dynamic data to be inside there. From the above example of /ski/:skiId, one actual endpoint might be something like /ski/1234 (where 1234 is the unique ID number of one of the skis in our database.
source: https://coursework.vschool.io/rest-api-design/#:~:text=%3AnounId%20%3A%20The%20colon%20(%3A)%20before,data%20to%20be%20inside%20there.

In URL `%` is replaced by `%25` when using `queryParams` while routing in Angular

I wanted to navigate to a URL using queryParams while Routing in Angular.
<a routerLink='/master' [queryParams]="{query:'%US',mode:'text'}"><li (click)="search()">Search</li></a>
The URL I wanted to navigate is:
http://localhost:4200/master?query=%US&mode=text
But when I click on search it navigates me to:
http://localhost:4200/master?query=%25US&mode=text
I do not know why 25 is appended after the % symbol. Can anyone tell me a cleaner way to navigate correctly.

In URLs, the percent sign has special meaning and is used to encode special characters. For example, = is encoded as %3D.
Certain special characters are not allowed in url. If you want to use those in url you have to encode them using encodeURIComponent javascript function.
%25 is actually encoded version of % character. Here browser is encoding them itself.
When trying to get queryParams from url , you can decode them using decodeURIComponent.
For more information check : https://support.microsoft.com/en-in/help/969869/certain-special-characters-are-not-allowed-in-the-url-entered-into-the
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/decodeURIComponent

# in URL skipping all parameter after that

When submitting form using get method if we pass # character in any field it skips all parameter after that field.
e.g.
bookmy_car.php?pod=6&room_id=32&starthour=14&startminute=00&startday=07&startmonth=08&startyear=2015&endhour=16&endminute=00&endday=07&endmonth=08&endyear=2015&end_date=1438927200&email_conf=1&cost_code=&desc=Trip description&trip_comment=#&day_rate=68.00&hourly_rate=6.60&hourly_km_rate=0.35&dur_hours=2
hours&location_charge=0.00&damage_cover_charge=5.00&total_free_kms=&longterm=0&rt=&minbooking=3600&returl=&returl_newid=&rep_id=&edit_type=&insPlanid=3&plan_name=goOccasional&id=3&driver_username_id=2&
How do we protect it? I tried escape() and encodeURI() function of JavaScript, it does not help.

I agree with #dgsq . But i prefer using only encodeURI so that he can get the uri as it is in the next page.
alert( encodeURI('&trip_comment=#&day_rate=68.00') )

It happens because with hashbang in query string # it is interpreted as location.hash and hot processed as GET parameters. You need to properly encode URI before you use it. For example with encodeURIComponent:
alert( encodeURIComponent('trip_comment=#') )

Problems with passing Euro Sign as URL parameter

I am trying to pass the Euro ( € ) sign as url parameter in my spring jsp. What is the right way to do so ? I tried the following with no avail. Problem is the character is getting encoded properly but not getting decoded from my destination jsp.
I am using
<%#page contentType="text/html;charset=UTF-8" %>
Here is the calling jsp:
<script>
...
// params contains the euro sign
document.location='dest.jsp?p='+escape(params);
In the dest.jsp
<input type="hidden" id="par" value="${param.p}">
and in a script in the same page
console.log($('#par').val())
when I use escape(params) I get the url as %u20AC . But no (empty) values in the dest.jsp
when I use encodeURI(params) or encodeURIComponent I get url as € . But the value in dest.jsp as ⬠- something which I can't use to render as euro sign

I'm going to assume you are using Tomcat because that's what I tested with and we get the same result.
What you will want to do is open up your Tomcat servlet.xml file and find the HTTP connector and add the useBodyEncodingForURI attribute with the value true.
<Connector connectionTimeout="20000" port="8080" protocol="HTTP/1.1"
redirectPort="8443" useBodyEncodingForURI="true">
</Connector>
Then, you will want to register a CharacterEncodingFilter to set the HttpServletRequest character encoding.
You can read more about this behavior in my answer here:
Character encoding in query string, hebrew

You need indeed to encode the € sign which should give %E2%82%AC using UTF-8. You need to be careful with the encoding you use on both ends.
Something like URLEncoder.encode(url, "UTF-8") on the client would do.
If you are using Spring, org.springframework.web.util.UriUtils has also nice utilities you can use.
If the decoding issue is on the server, you need first to make sure that your web container decodes the URI with the proper encoding.
Tomcat decodes URI with ISO-8859-1 by default so you need to update your connector configuration
<Connector port="8080" ...
URIEncoding="UTF-8"/>

See the following answers
Spring MVC: How to store € character?
Getting question mark instead accented letter using spring MVC 3
I think that org.springframework.web.filter.CharacterEncodingFilter should help here.
Try it with and without your encodeURI(params)

Why use encodeURIComponent() when writing json to a cookie

In particular, when saving a JSON to the cookie is it safe to just save the raw value?
The reason I dopn't want to encode is because the json has small values and keys but a complex structure, so encoding, replacing all the ", : and {}, greatly increases the string length

if your values contain "JSON characters" (e.g. comma, quotes, [] etc) then you should probably use encodeURIComponent so these get escaped and don't break your code when reading the values back.

You can convert your JSON object to a string using the JSON.stringify() method then save it in a cookie.
Note that cookies have a 4000 character limit.
If your Json string is valid there should be no need to encode it.
e.g.
JSON.stringify({a:'foo"bar"',bar:69});
=> '{"a":"foo\"bar\"","bar":69}' valid json stings are escaped.

This is documented very well on MDN
To avoid unexpected requests to the server, you should call encodeURIComponent on any user-entered parameters that will be passed as part of a URI. For example, a user could type "Thyme &time=again" for a variable comment. Not using encodeURIComponent on this variable will give comment=Thyme%20&time=again. Note that the ampersand and the equal sign mark a new key and value pair. So instead of having a POST comment key equal to "Thyme &time=again", you have two POST keys, one equal to "Thyme " and another (time) equal to again.

If you can't be certain that your JSON will not include reserved characters such as ; then you will want to perform escaping on any strings being stored as a cookie. RFC 6265 covers special characters that are not allowed in the cookie-name or cookie-value.
If you are encoding static content you control, then this escaping may be unnecessary. If you are encoding dynamic content such as encoding user generated content, you probably need escaping.
MDN recommends using encodeURIComponent to escape any disallowed characters.
You can pull in a library such as cookie to handle this for you, but if your server is written in another language you will need to ensure it uses a library or language utilities to encodeURIComponent when setting cookies and to decodeURIComponent when reading cookies.
JSON.stringify is not sufficient as illustrated by this trivial example:
const bio = JSON.stringify({ "description": "foo; bar; baz" });
document.cookie = `bio=${stringified}`;
// Notice that the content after the first `;` is dropped.
// Attempting to JSON.parse this later will fail.
console.log(document.cookie) // bio={\"description\":\"foo;

Cookie: name=value; name2=value2
Spaces are part of the cookie separation in the HTTP Cookie header. Raw spaces in cookie values could thus confuse the server.