REST API to upload file in doc lib

Hope you have idea how to create SharePoint App.
If you are using NAPA tool then replace below codes in APP.js file.
So i am just focusing on the REST script part to do the activities

I recommend to go through REST API for SP document libraries before proceed.

HTML file in Default.aspx 

1. <input id="getFile" type="file"/><br />  
2. <input id="displayName" type="text" value="Enter a unique name" /><br />  
3. <input id="addFileButton" type="button" value="Upload" onclick="uploadFile()"/>  

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JS script in app.js (Paste the below methods in app.js file) Refer here
-----------------------
function uploadFile()
{
 
    // Define the folder path for this example.
    var serverRelativeUrlToFolder = '/sites/apps/shared documents';
 
    // Get test values from the file input and text input page controls.
    // The display name must be unique every time you run the example.
    var fileInput = $('#getFile');
    var newName = $('#displayName').val();
 
    // Initiate method calls using jQuery promises.
    // Get the local file as an array buffer.
    var getFile = getFileBuffer();
    getFile.done(function (arrayBuffer) {
 
        // Add the file to the SharePoint folder.
        var addFile = addFileToFolder(arrayBuffer);
        addFile.done(function (file, status, xhr) {
 
            // Get the list item that corresponds to the uploaded file.
            var getItem = getListItem(file.d.ListItemAllFields.__deferred.uri);
            getItem.done(function (listItem, status, xhr) {
 
                // Change the display name and title of the list item.
                var changeItem = updateListItem(listItem.d.__metadata);
                changeItem.done(function (data, status, xhr) {
                    alert('file uploaded successfully in Library);
                });
                changeItem.fail(onError);
            });
            getItem.fail(onError);
        });
        addFile.fail(onError);
    });
    getFile.fail(onError);
 
    // Get the local file as an array buffer.
    function getFileBuffer()  
    {
        var deferred = jQuery.Deferred();
        var reader = new FileReader();
        reader.onloadend = function (e) {
            deferred.resolve(e.target.result);
        }
        reader.onerror = function (e) {
            deferred.reject(e.target.error);
        }
        reader.readAsArrayBuffer(fileInput[0].files[0]);
        return deferred.promise();
    }
 
    // Add the file to the file collection in the Shared Documents folder.
    function addFileToFolder(arrayBuffer)  
    {
 
        // Get the file name from the file input control on the page.
        var parts = fileInput[0].value.split('\\');
        var fileName = parts[parts.length - 1];
 
        // Construct the endpoint.
        var fileCollectionEndpoint = String.format(
            "{0}/_api/sp.appcontextsite(@target)/web/getfolderbyserverrelativeurl('{1}')/files" +
            "/add(overwrite=true, url='{2}')?@target='{3}'",
            appWebUrl, serverRelativeUrlToFolder, fileName, hostWebUrl);
 
        // Send the request and return the response.
        // This call returns the SharePoint file.
        return $.ajax({
            url: fileCollectionEndpoint,
            type: "POST",
            data: arrayBuffer,
            processData: false,
            headers: {
                "accept": "application/json;odata=verbose",
                "X-RequestDigest": jQuery("#__REQUESTDIGEST").val(),
                "content-length": arrayBuffer.byteLength
            }
        });
    }
 
    // Get the list item that corresponds to the file by calling the file's ListItemAllFields property.
    function getListItem(fileListItemUri)  
    {
 
        // Construct the endpoint.
        // The list item URI uses the host web, but the cross-domain call is sent to the
        // app web and specifies the host web as the context site.
        fileListItemUri = fileListItemUri.replace(hostWebUrl, '{0}');
        fileListItemUri = fileListItemUri.replace('_api/Web', '_api/sp.appcontextsite(@target)/web');
         
        var listItemAllFieldsEndpoint = String.format(fileListItemUri + "?@target='{1}'",
            appWebUrl, hostWebUrl);
 
        // Send the request and return the response.
        return $.ajax({
            url: listItemAllFieldsEndpoint,
            type: "GET",
            headers: { "accept": "application/json;odata=verbose" }
        });
    }
 
    // Change the display name and title of the list item.
    function updateListItem(itemMetadata)
    {
 
        // Construct the endpoint.
        // Specify the host web as the context site.
        var listItemUri = itemMetadata.uri.replace('_api/Web', '_api/sp.appcontextsite(@target)/web');
        var listItemEndpoint = String.format(listItemUri + "?@target='{0}'", hostWebUrl);
 
        // Define the list item changes. Use the FileLeafRef property to change the display name.  
        // For simplicity, also use the name as the title.
        // The example gets the list item type from the item's metadata, but you can also get it from the
        // ListItemEntityTypeFullName property of the list.
        var body = String.format("{{'__metadata':{{'type':'{0}'}},'FileLeafRef':'{1}','Title':'{2}'}}",
            itemMetadata.type, newName, newName);
 
        // Send the request and return the promise.
        // This call does not return response content from the server.
        return $.ajax({
            url: listItemEndpoint,
            type: "POST",
            data: body,
            headers: {
                "X-RequestDigest": jQuery("#__REQUESTDIGEST").val(),
                "content-type": "application/json;odata=verbose",
                "content-length": body.length,
                "IF-MATCH": itemMetadata.etag,
                "X-HTTP-Method": "MERGE"
            }
        });
    }
}
 
// Display error messages.  
function onError(error)  
{
    alert(error.responseText);
}