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Checkbox fields

A checkbox field stores a string as its value, and a string as its text. Its value is either 'TRUE' or 'FALSE', and its text is either 'true' or 'false'.

Checkbox field

Checkbox field on collapsed block



  "type": "example_checkbox",
  "message0": "checkbox: %1",
  "args0": [
      "type": "field_checkbox",
      "name": "FIELDNAME",
      "checked": true


Blockly.Blocks['example_checkbox'] = {
  init: function() {
        .appendField(new Blockly.FieldCheckbox(true), 'FIELDNAME');

The checkbox constructor takes in an optional value and an optional validator. The optional value should be either 'TRUE', 'FALSE', or a boolean, otherwise it will default to false.

Serialization and XML

The XML for a checkbox field looks like so:

<field name="FIELDNAME">TRUE</field>


<field name="FIELDNAME">true</field>

The name attribute contains a string referencing a checkbox field, and the inner text is the field's value. The inner text value follows the same rules as the constructor value.

Note that after being deserialized and reserialized all of the inner text values will be in caps ('TRUE' or 'FALSE'). This is sometimes important when diffing workspaces.


Checkmark character

The Blockly.FieldCheckbox.CHECK_CHAR property can be used to change what the checkmark looks like. The value should be a string containing a unicode character.

Checkbox field with heart instead of check

The CHECK_CHAR property defaults to '\u2713' or ✓.

This is a global property, so it will modify all checkbox fields when set.

Creating a checkbox validator

A checkbox field's value is either 'TRUE' or 'FALSE' so a validator should accept those values (i.e. a string) and return 'TRUE', 'FALSE', null, or undefined.

Here's an example of a validator that hides or shows a text input field based on whether the checkbox is checked:

  validate: function(newValue) {
    var sourceBlock = this.getSourceBlock();
    sourceBlock.showTextField_ = newValue == 'TRUE';

    return newValue;

  updateTextField: function() {
    var input = this.getInput('DUMMY');
    if (this.showTextField_ && !this.getField('TEXT')) {
      input.appendField(new Blockly.FieldTextInput(), 'TEXT');
    } else if (!this.showTextField_ && this.getField('TEXT')) {

Checkbox field with a validator