Difference between revisions of "GM.getValue"

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(Update for 4.0)
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Retrieving the value associated with the name 'foo':
Retrieving the value associated with the name 'foo':
<pre class='sample'>
<pre class='sample-good'>
console.log(await GM.getValue("foo"));  
console.log(await GM.getValue("foo"));  
</pre>
</pre>
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A more complex example:  
For structured data:
 
Used <code>JSON.stringify()</code> to place an object into storage and then <code>JSON.parse()</code> to convert it back.
If you have used JSON.stringify to place an object into storage, JSON.parse shall be used to convert it back from a string to a Javascript Object.


<pre class='sample'>
<pre class='sample'>
//Note that if the value associated with foo is an invalid
//Note that if the value associated with foo is an invalid
//  JSON string, JSON.parse will fail.
//  JSON string, JSON.parse will fail.
//  Also Note the default value is in quotes (thus a string) rather than an object literal
//  Also note the default value is in quotes (thus a string) rather than an object literal


var storedObject = JSON.parse(await GM.getValue("foo", "{}"));  
var storedObject = JSON.parse(await GM.getValue("foo", "{}"));  
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}
}
</pre>
</pre>
Complete end-to-end set and get example:
<pre class='sample-good'>
// ==UserScript==
// @name        Greasemonkey set-and-get Example
// @description Stores and logs a counter of executions.
// @grant      GM.setValue
// @grant      GM.getValue
// ==/UserScript==
(async () => {
  let count_before = await GM.getValue('count', 0);
 
  // Note awaiting the set -- required so the next get sees this set.
  await GM.setValue('count', count_before + 1);
  // Get the value again, just to demonstrate order-of-operations.
  let count_after = await GM.getValue('count');
 
  console.log('Greasemonkey set-and-get Example has run', count_after, 'times');
})();
</pre>
Doing many gets/many sets can be slow.
Instead get/set one value (i.e. like with <code>JSON.stringify()</code> above) or use [https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all Promise.all()] to start all operations in parallel, then wait for all of them to complete.


== See Also ==
== See Also ==

Revision as of 14:18, 23 November 2017

Description

This method retrieves a value that was set with GM.setValue. See GM.setValue for details on the storage of these values.

Syntax

function GM_getValue( name, default )

Compatibility: Greasemonkey 4.0+

Arguments

name
String The property name to get. See GM.setValue for details.
default
Optional. The default value to be returned when none has previously been set.

Returns

A Promise, rejected in case of error and otherwise resolved with:

When this name has been set
String, Integer or Boolean as previously set
When this name has not been set, and default is provided
The value passed as default
When this name has not been set, and default is not provided
undefined

Examples

Retrieving the value associated with the name 'foo':

console.log(await GM.getValue("foo")); 

Retrieving the value associated with the name 'timezoneOffset' with a default value defined:

//GM_getValue() returns the value -5 (integer) if no value 
//   associated with the name timezoneOffset is found in storage

console.log(await GM.getValue("timezoneOffset", -5)); 


For structured data: Used JSON.stringify() to place an object into storage and then JSON.parse() to convert it back.

//Note that if the value associated with foo is an invalid
//   JSON string, JSON.parse will fail.
//   Also note the default value is in quotes (thus a string) rather than an object literal

var storedObject = JSON.parse(await GM.getValue("foo", "{}")); 

if (!storedObject) {
  //JSON.parse() should never return any value that type-casts to false, assume there is an 
  //   error in the input string
  console.warn('Error! JSON.parse failed - The stored value for "foo" is likely to be corrupted.');
  throw;
}

Complete end-to-end set and get example:

// ==UserScript==
// @name        Greasemonkey set-and-get Example
// @description Stores and logs a counter of executions.
// @grant       GM.setValue
// @grant       GM.getValue
// ==/UserScript==

(async () => {
  let count_before = await GM.getValue('count', 0);
  
  // Note awaiting the set -- required so the next get sees this set.
  await GM.setValue('count', count_before + 1);

  // Get the value again, just to demonstrate order-of-operations.
  let count_after = await GM.getValue('count');
  
  console.log('Greasemonkey set-and-get Example has run', count_after, 'times');
})();

Doing many gets/many sets can be slow. Instead get/set one value (i.e. like with JSON.stringify() above) or use Promise.all() to start all operations in parallel, then wait for all of them to complete.

See Also