Zeal allows you to consult easily and quickly the documentation of different programming languages.

Programming languages consist of multiple functions, classes, expressions, etc; this is known as the API (Application Programming Interface). A programmer must know well the programming language's API. To do this, you can query the programming language Online documentation. This involves a series of drawbacks, such as being limited by the online search engine of the API or depending on the connection to the Internet to browse the programming language API.

Zeal arises as a solution that allows us to query the APIs of different programming languages in an organized and systematic way.
This application can download the APIs documentation from many programming languages, such as PHP, Javascript, Java, Python, Ruby, Scala, Swift, C/C++, ASP, Clojure, Go, Perl, etc.
In addition, we will also be able to download the APIs from different frameworks, including Django, Symfony, Laravel, Ruby on Rails, Yii, etc.
We also can access other APIs such as HTML, CSS, Joomla, Drupal, Emmet, OpenCV, PhoneGap, MySQL, Unity 3D, etc.

We must download the APIs documentation to make use of them. This is a great advantage because once downloaded to our computer; there is no need to be connected to the Internet to query the documentation.
You must follow these steps to download the APIs documentation: Edit->Options->Docsets . At the bottom, you will see a list with all the available APIs ready to be downloaded. We will select the docs we're interested in and click on the “ Download ” button.

Once we have downloaded the documents from the APIs we are interested in, we will see all the downloaded APIs on the left side of the main screen.
We will be able to navigate through the APIs as if they were directories in such a way that we will be able to see a list with all the modules, methods, classes, exceptions, etc., of the APIs.
If you select any documents in the left column, you will see the information on this document in the right column. This document is like a web page; we can click on the links to continue reading information related to this document. Pressing Ctrl+F , we can search for any word within the selected document.
The program has a global search engine located at the top left column. This search engine searches all the downloaded APIs.
If we want to search in a single document, we must enter the name of the programming language plus a colon, followed by the word we want to search. For example, if we want to search for information about the javascript function “map,” we must enter the following on the search box: javascript:map

To launch the program, we can configure a keyboard shortcut ( Alt+space bar ) so the program is always at hand, and we can query the APIs quickly.

Although the program is very useful and comfortable due to the combination of keys, we can make the most of its functionality and integrate it with our favorite editors, such as Sublime Text, Emacs, Vim, Brackets, or IntelliJ IDEA.
For example, suppose we install the Sublime Text plugin that allows the integration of Zeal. In that case, we can press the Shift+F1 key combination and use the Sublime Text search box to search for the downloaded APIs documentation.
We can also select a word on the code we are writing, and by pressing the F1 key, Zeal will be launched and will show the related documentation. For example, if we edit an HTML document, select the word “href,” and press F1 , we will see the information related to that HTML attribute.

This program is free and is available for Windows and Linux.