JavaScript Field

Table of contents

The JavaScript field is similar to a calculated field, but uses JavaScript expressions for calculations. Unlike other scripts in Memento Database, this field executes only in the context of an entry (has access to entry fields), not to any data. You can only read field values - modification of field values is not supported.

A script may consist of multiple operations, but only the result of the last operation will be entered into the field. For example:

var i = 1 + 1  // Result would be 2, if this were the final statement,
i + 1          //   but instead, this is the final statement, so the result will be 3

Accessing Entry Fields

Use the field("fieldName") function to access values of entry fields. The type of JavaScript object returned depends on the field type.

Field Type Compatibility

Memento Field Type JavaScript Object Type Notes
Text String Direct text value
Number Number Numeric value
Currency Number Numeric value without currency symbol
Date/Time Date JavaScript Date object
Checkbox Boolean true/false value
Rating Number Numeric rating value
Link to Entry Array Array of linked entry objects
File/Image Object File metadata object
Location Object Geographic coordinates object

Script Execution Timing

A script can be executed in two scenarios:

  1. On Edit (Default): Script runs only when an entry is created or modified
    • Better performance for large libraries
    • Suitable for calculations that don’t depend on current time
  2. Real-time: Script runs both on edit and when displayed
    • Enable this option if your script contains functions that depend on current time
    • May significantly increase loading time for libraries with many entries

Basic Examples

Example: Product Total Price

Library contains fields: Price and Count JavaScript field Total Price:

field('Price') * field('Count');

Example: Percentage Calculation

Library contains fields: Count and Total JavaScript field Percentage:

var total = field('Total');
if (total > 0)
   field('Count') * 100 / total + ' %';
else
   'None';

Example: Days Between Dates

Library contains DateTime fields: Start and End JavaScript field Days:

moment(field('End')).diff(field('Start'), 'days');

Example: Age-Based Classification

Library contains a Birthday field. Display “Youth” for people under 18, otherwise “Adult”:

var years = moment().diff(field('Birthday'), 'years');
if (years < 18)
   'Youth';
else
   'Adult';

Working with Linked Entries

When you have a many-to-many Link To Entry field linking the current entry to entries in another library, you can perform calculations across those linked entries.

Example: Sum of Linked Amounts 

var amountField = "Amount";        // CHANGE Amount to your amount field name
var linkedEntries = field("Them"); // CHANGE Them to your link field name
var numLE = linkedEntries.length;  // The number of linked entries

// Loop through linked entries, creating the sum
for (var lex = 0, result = 0; lex < numLE; lex++)
    result += linkedEntries[lex].field(amountField);

result;                            // The result is the SUM

Example: Average of Linked Amounts 

var amountField = "Amount";        // CHANGE Amount to your amount field name
var linkedEntries = field("Them"); // CHANGE Them to your link field name
var numLE = linkedEntries.length;  // The number of linked entries

// Loop through linked entries, creating the sum
for (var lex = 0, result = 0; lex < numLE; lex++)
    result += linkedEntries[lex].field(amountField);

if (numLE != 0)
    result / numLE;                // The result is the AVERAGE

Example: Minimum of Linked Amounts 

var amountField = "Amount";        // CHANGE Amount to your amount field name
var linkedEntries = field("Them"); // CHANGE Them to your link field name
var numLE = linkedEntries.length;  // The number of linked entries
var temp;

// Loop through linked entries, finding the MINIMUM
for (var lex = 0, result = 0; lex < numLE; lex++) {
    temp = linkedEntries[lex].field(amountField);
    if (temp < result)
        result = temp;
}

result;                            // The result is the MINIMUM

Example: Maximum of Linked Amounts 

var amountField = "Amount";        // CHANGE Amount to your amount field name
var linkedEntries = field("Them"); // CHANGE Them to your link field name
var numLE = linkedEntries.length;  // The number of linked entries
var temp;

// Loop through linked entries, finding the MAXIMUM
for (var lex = 0, result = 0; lex < numLE; lex++) {
    temp = linkedEntries[lex].field(amountField);
    if (temp > result)
        result = temp;
}

result;                            // The result is the MAXIMUM

Example: Order Cost Calculation

Library Orders contains orders, library Items contains items. Orders has a link field Items to the Items library. Items contains fields Quantity and Price.

To calculate order cost, create a JavaScript field Cost:

var items = field('Items');        // CHANGE Items to your link field name
var cost = 0;
for (var i in items)
    cost += items[i].field('Quantity') * items[i].field('Price');

cost;                              // The result is the COMPUTED COST

Working with Dates - moment.js

Enable the integrated momentjs.com library for advanced date and time operations. In the field configuration, click JavaScript Libraries and select moment.min.js.

Date Format Examples

If an entry contains a Date field with value “18 March 2016 14:55”:

moment(field('Date')).format('MMMMM Do YYYYY')      // March 18th 2016
moment(field('Date')).format('dddd')                // Friday
moment(field('Date')).format('dddd [Escaped] YYYY') // Friday Escaped 2016
moment(field('Date')).format('HH:mm')               // 14:55

Relative Time Examples 

moment(field('Date')).fromNow()                     // 11 minutes ago
moment(field('Date')).startOf('day').fromNow()      // 15 hours ago

Calendar Time Examples 

moment(field('Date')).calendar()                    // Today at 2:55 PM
moment(field('Date')).add(1, 'days').calendar()     // Tomorrow at 2:55 PM

Best Practices

  1. Performance: Use “Execute script in real time: No” unless you need current time-dependent calculations
  2. Error Handling: Include checks for undefined or null values when accessing fields
  3. Naming: Use descriptive variable names and add comments for complex calculations
  4. Testing: Test your scripts with various data scenarios to ensure reliability
  5. Optimization: Keep calculations simple and efficient for better performance