I really dislike when ServiceNow doesn't share the functions they make
available and use in their code in front of us. See this thread on the
community. Anyways,
there are efforts to keep track of these things. I know I've contributed
to this page on
serviceportal.io.
s You can get the list of all the methods available by running this in
Xplore;

var sp = new GlideSPScriptable();
sp;

canReadRecord

// Returns true if the user can read the specified GlideRecord.
// source: SQANDA Create Question[sqanda-create-question] line 40:
if (!$sp.canReadRecord(questionGR)){

}
// source: SC Popular Items[cbcacf13cb21020000f8d856634c9ca0] line 14:
$sp.canReadRecord(
   "sc_cat_item",
   count.cat_item.sys_id.getDisplayValue()
);

getCatalogItem

// Returns a model and view model for a sc_cat_item or
// sc_cat_item_guide.
// source: SC Catalog Item[widget-sc-cat-item] line 50
data.sc_cat_item = $sp.getCatalogItem(data.sys_id, true);
// source: sp-variable-editor[sp-variable-editor] line 25
data.sc_cat_item = $sp.getCatalogItem(
    gr.cat_item,
    !!options.isOrdering,
    targetTable
);

getKBCategoryArticleSummaries

getKBCategoryArticleSummaries(sys_id, number,
maxChars)

Returns Knowledge Base article summaries in the specified category and
its subcategories.

// KB Categories - KBv3[ee307070d7201200a9addd173e24d4da]
/* Line 17: */        if (kbCount < 500) { // if more than 500 articles, don't iterate to get viewable counts
/* Line 18: */          var arts = $sp.getKBCategoryArticleSummaries(cats.getUniqueValue(), 0, 0);
/* Line 19: */          articleCount = arts.length;

// KB Category Page[fb5d068cd7610200a9ad1e173e24d400]
/* Line 13: */  data.categoryDisplay = kb_cat.getDisplayValue();
/* Line 14: */  data.items = $sp.getKBCategoryArticleSummaries(data.category, 0, 250);
/* Line 15: */}

getDisplayValue

getDisplayValue(stringfield)

// Returns a display value from a field on a record in this order:
// 1. The widget's sp_instance* record
// 2.
// source: SC Catalog Item[widget-sc-cat-item] line 4
data.sc_catalog_page = $sp.getDisplayValue("sc_catalog_page");

getField

getField(GlideRecord, string
field)

// Returns {display_value, label, type, value} for a given field
// on a GlideRecord.
// source: Approval Info[33442352cb30020000f8d856634c9c3f] line 21
fields.push($sp.getField(
    gr,
    'sys_updated_on'
));

getFields

getFields(GlideRecord, string
fields)

// Like getField Checks the specified field names, and returns
// a comma seperated list of valid names.
// source: Approval Info[33442352cb30020000f8d856634c9c3f] line 18
var fields = $sp.getFields(
    gr,
    'state,sys_created_on'
);

getFieldsObject

getFieldsObject(GlideRecord, string
fields)

// Checks the specified field names, and returns an object
// containing the valid names.
// source: Approval Record[525a2752cb30020000f8d856634c9cfd] line 15
var t = {};
t = $sp.getFieldsObject(
    task,
    'number,short_description,opened_by,requested_by,start_date'
);

getForm

// Returns the form.
// If no view is passed, returns the form as it appears on view 'sp'
// source: User Profile[user-profile] line 22
$sp.getForm("live_profile", data.liveProfileID);
// source: Form[widget-form] line 63
data.f = $sp.getForm(data.table, data.sys_id, data.query, data.view);

getListColumns

getListColumns(string table, string
view)

// Returns a list of the specified table's columns in the specified
// view
// source: Data Table[widget-data-table] line 37
data.fields = $sp.getListColumns(data.table, data.view);

getMenuHREF

getMenuHREF(GlideRecord)

// Returns the (?id=) portion of the URL based on the sp_menu type.
// source: Icon Link[3caa67afcb13020000f8d856634c9c2e] line 3
data.href = $sp.getMenuHREF(gr);

getMenuItems

getMenuItems(string
sys_id)

// Returns the menu items for the specified instance.
// source Header Menu[5ef595c1cb12020000f8d856634c9c6e] line 8
data.menu.items = $sp.getMenuItems(menu_id);

getParameter

getParameter(string)

// Returns the value of a given key from the query string or post body.
// source: Form[widget-form] line 37
data.query = $sp.getParameter("query") || options.query;

getPortalRecord

getPortalRecord()

// Returns the portal's GlideRecord.
// source: Subscribed Questions[sqanda-subscribed-questions] line 2
data.knowledgeBase = $sp.getPortalRecord().getValue("sqanda_knowledge_base");

getRecord

// Returns the GlideRecord for the current sp_instance\*.
// Returns null if the widget is embedded by another widget.
// source: Request Fields[96cf39f3d7230200a9addd173e24d412] line 2
var gr = $sp.getRecord();
// source: Form[widget-form] line 54
var rec = $sp.getRecord(data.table, data.sys_id);

Note: Feedback submitted on 04/24/2018 page:
https://developer.servicenow.com/app.do#!/api_doc?v=jakarta&id=r_GSPS-getRecord

What are you trying to do now? Look up information
Were you successful? no
Tell us more
$sp.getRecord is missing information.
getRecord(string table, string sys_id) as depicted in OOB widget `widget-form` line54
var rec = $sp.getRecord(data.table, data.sys_id);

This however doesnt work in scoped apps.

getRecordDisplayValues

getRecordDisplayValues(object data, GlideRecord, string
fields)

// Copies display values for the specified field names
// from a GlideRecord into the data parameter.
// source: Catalog Content[sc-cat-content] line 26
$sp.getRecordDisplayValues(c, gr, 'picture');

getRecordElements

getRecordElements(object data, GlideRecord, string
fields)

// Copies the value and display value for the specified
// field names from a GlideRecord into the data parameter.
// source: Data Table[widget-data-table] line 128
$sp.getRecordElements(record, gr, data.fields);

getRecordValues

getRecordValues(object data, GlideRecord, string
fields)

// Copies values for the specified field names from a
// GlideRecord into the data parameter.
// source: KB View 2[c254be50d7201200a9addd173e24d410] line 6
$sp.getRecordValues(data, gr, 'title,description');

getStream

getStream(string table, string
sys_id)

// Get the activity stream for a record.
// source: Ticket Conversations[widget-ticket-conversation] line 63
data.stream = $sp.getStream(data.table, data.sys_id);

Note: This only returns the quantity of entries specified by the
property, glide.service_portal.stream_entry_limit. Default is 100.

getUserInitials

getUserInitials()

// Returns the user's initials as a string.

getValue

getValue(string
field)

// Like getDisplayValue except that it returns the value instead of
// the display value.
// source: SC Popular Items[cbcacf13cb21020000f8d856634c9ca0] line 1
data.sc_catalog = $sp.getValue('sc_catalogs') || $sp.getValue('sc_catalog');

getValues

//source: KB Topic Articles[35c51f56d7f33100a9ad1e173e24d4b1] line 2
$sp.getValues(data, "color,glyph,kb_topic,title");
//source: Icon Menu List[88979930cb01020000f8d856634c9caa] line 2
$sp.getValues(data);

getWidget

getWidget(string sys_id/widget-id, object
options)

// Returns a widget model for embedding a widget inside another widget.
// source: User Profile[user-profile] line 75
data.teamWidget = $sp.getWidget(
    'sp-my-team',
    {glyph: 'user', color: 'primary'}
);

mapUrlToSPUrl

mapUrlToSPUrl()

/* Line 98:  */    if (redirectURL && redirectURL != "true") {
/* Line 99:  */        var spUrl = new GlideSPScriptable().mapUrlToSPUrl(redirectURL);
/* Line 100: */        returnUrl = spUrl ? this.portal + "?" + spUrl : redirectURL;

isServicePortalURL

isServicePortalURL()

/* Line 65: */      var nt = session.getProperty("nav_to");
/* Line 66: */      var isServicePortalURL = new GlideSPScriptable().isServicePortalURL(nt);
/* Line 67: */      var redirectURL = session.getProperty("login_redirect");
/* Line 68: */
/* Line 69: */      if (user.hasRoles() && !redirectURL && !isServicePortalURL)
/* Line 70: */          return;

translateTemplate

translateTemplate()

/* Line 27: */          if (searchGroupTemplatesGR.advanced_typeahead_config)
/* Line 28: */              data.typeaheadTemplates["sp-typeahead-" + searchGroupTemplatesGR.getValue("id") + ".html"] = $sp.translateTemplate(searchGroupTemplatesGR.getValue("typeahead_template"));
/* Line 29: */      }

stripHTML

stripHTML()

/* Line 21: */      if (!!instance.description)
/* Line 22: */          instance.description = $sp.stripHTML(instance.description);
/* Line 23: */  });

getRelatedList

getRelatedList()

/* Line 1:*/ data.slides = $sp.getRelatedList('sp_carousel_slide','carousel');

getGuide

getGuide()

/*Line 152:*/   data.showPrices = $sp.showCatalogPrices();
/*Line 153:*/   data.sc_cat_item = $sp.getGuide(data.sys_id, true, false);
/*Line 154:*/   if (data.sc_cat_item.category) {

getKBRecord

getKBRecord()

// Returns the portal's KB record where the workflow state is
// published.
// source: KB Most Viewed[7d903b52cb30020000f8d856634c9ce3] line 4
var z = $sp.getKBRecord();

getKBCount

getKBCount(string
sys_id)

// Returns the number of Knowledge Base articles.
// source: KB Categories - KBv3[ee307070d7201200a9addd173e24d4da] line 16
var kbCount = $sp.getKBCount(data.kb);

getKBCategoryArticles

getKBCategoryArticles(string
sys_id)

// Returns KB articles in the specified category and its
// subcategories.Pass 'limit' to limit the number of
// articles returned.
// source: KB Categories[122ac7f0d7101200a9addd173e24d411] line 9
var articles = $sp.getKBCategoryArticles(t.getUniqueValue());

getKBTopicArticles

getKBTopicArticles(string
topic)

// Returns array
// source: KB Topic Articles[35c51f56d7f33100a9ad1e173e24d4b1] line 3
data.items = $sp.getKBTopicArticles(data.kb_topic);

getKBTopCategoryID

getKBTopCategoryID(string
sys_id)

// Returns the top category in the hierarchy containing the
// specified category.
// source: KB View 2[c254be50d7201200a9addd173e24d410] line 22
n.topCat = $sp.getKBTopCategoryID(n.kb_category);

getKBSiblingCategories

getKBSiblingCategories(string
sys_id)

// Returns KB categories with same parent as the specified
// category.
// source: KB Categories - KBv3[ee307070d7201200a9addd173e24d4da] line 6
cats = $sp.getKBSiblingCategories(data.catParam);

showCatalogPrices

showCatalogPrices()

// Returns- Is an object|
// source: SC Catalog Item[widget-sc-cat-item] line 20
data.showPrices = $sp.showCatalogPrices();

saveVariables

saveVariables()

// Retuns ?
// source: sp-variable-editor[sp-variable-editor] line 17
$sp.saveVariables(
    input.table,
    input.sys_id,
    vars
);

buildThemeVariableModel

buildThemeVariableModel(string sys_id,
array)

// Retuns ?
// source: Portal config[widget-portal-config] line 241
var variableValues = $sp.buildThemeVariableModel(portalId, keys.join(","));

getVariablesArray

getVariablesArray()

// Returns the tables variables as an object.
// source: Request Fields[96cf39f3d7230200a9addd173e24d412] line 22
data.variables = $sp.getVariablesArray();

getWidgetFromInstance

getWidgetFromInstance(string)

// Returns a widget from the specified widget instance.
// source: KB Search[9b6187d0d7201200a9addd173e24d4dd] line 1
data.typeahead = $sp.getWidgetFromInstance('typeahead_search_header');

getRecordVariablesArray

getRecordVariablesArray(GlideRecord)

// Returns the records variables.
// source: Approvals[f37aa302cb70020000f8d856634c9cfc] line 86
item.variables = $sp.getRecordVariablesArray(itemsGR);

getFilterBreadcrumbs

getFilterBreadcrumbs(string table, string, query,
null)

// Returns an array of objects where each object contains
// the breadcrumb label, value, and flags for if fixed and
// if removed
// source: Filter Breadcrumbs[widget-filter-breadcrumbs] line 4
data.breadcrumbs = $sp.getFilterBreadcrumbs(table, query, null);

logStat

// Create a new entry in the `sp_log` table.
// Record has a table name, a record sys_id from that name,
// and some type and optional comments.
// Handy for doing things like logging searches or visits
// to pages, etc.
// source: SC Catalog Item[widget-sc-cat-item] line 59
$sp.logStat(
    'Catalog View',
    data.sc_cat_item.sys_class_name,
    data.sys_id,
    data.sc_cat_item.name
);
// source: SC Order Guide[widget-sc-order-guide] line 38
$sp.logStat(
    'Catalog Order Guide View',
    data.sc_cat_item.sys_class_name,
    data.sys_id
);

getInstanceRecord

getInstanceRecord()

// Returns the widget instances GlideRecord.
// source: Icon Link[3caa67afcb13020000f8d856634c9c2e] line 2
var gr = $sp.getInstanceRecord();

log

// Sends the specified message to the log console.
// source: Calculator[calc] line 7
$sp.log("calc with input as follows... ");
// source: Preview Widget[core-preview] line 21
$sp.log(e);

saveRecord

saveRecord(string table, string sys_id, array fields)

// Saves or updates the current record.
// source: Form[widget-form] line 25
result = $sp.saveRecord(
    input.table,
    input.sys_id,
    input._fields
);

logSearch

logSearch()

// Adds a record to the Service Portal Statistics logs.
// no examples in a new instance

getSCRecord

getSCRecord()

// Returns sc_cat_item record for the portal's catalog with
// sys_class_name != sc_cat_item_wizard and active = true
// in the query. GlideRecord returned has not yet triggered
// the query.
// no examples in a new instance

getStreamEntries

getStreamEntries()

// Get a record's actvivity stream as a JSON formatted list.
// Typically for a task record