What causes "initialize must be a function" in phaserjs/phaser?

Ensure definition.initialize is a function declaration or expression: `initialize: function () { ... }`.

phaserjs/phaser · error · Error

initialize must be a function

Error message

initialize must be a function

What it means

Thrown by the Phaser.Class factory when the definition object has an `initialize` key whose value is not a function. `initialize` is the reserved slot for the ES5-style constructor; Phaser requires it to be callable so it can be invoked on instantiation. Any other type (string, number, object, array, null when truthy-checked via property access) is rejected up front rather than failing later at `new`.

Source

Thrown at src/utils/Class.js:193

 *          }
 *      });
 */
function Class (definition)
{
    if (!definition)
    {
        definition = {};
    }

    //  The variable name here dictates what we see in Chrome debugger
    var initialize;
    var Extends;

    if (definition.initialize)
    {
        if (typeof definition.initialize !== 'function')
        {
            throw new Error('initialize must be a function');
        }

        initialize = definition.initialize;

        //  Usually we should avoid 'delete' in V8 at all costs.
        //  However, its unlikely to make any performance difference
        //  here since we only call this on class creation (i.e. not object creation).
        delete definition.initialize;
    }
    else if (definition.Extends)
    {
        var base = definition.Extends;

        initialize = function ()
        {
            base.apply(this, arguments);
        };
    }

View on GitHub (pinned to 41be1e462b)

Solutions

Ensure definition.initialize is a function declaration or expression: `initialize: function () { ... }`.
If you do not need a custom constructor, omit initialize entirely; Phaser will synthesize one (delegating to Extends if present).
Check for typos: the key must be exactly 'initialize' (American spelling).
If migrating from ES6 class, map `constructor` to `initialize` explicitly.

Example fix

// before
var Enemy = new Phaser.Class({
  initialize: 'spawnEnemy',
  hp: 10
});

// after
var Enemy = new Phaser.Class({
  initialize: function (hp) { this.hp = hp; },
  hp: 10
});

Defensive patterns

Strategy: type-guard

Validate before calling

function safePhaserClass(definition) {
  if (definition && 'initialize' in definition && typeof definition.initialize !== 'function') {
    delete definition.initialize;
  }
  return new Phaser.Class(definition);
}

Type guard

function isValidClassDefinition(d) {
  return !d || !('initialize' in d) || typeof d.initialize === 'function';
}

Try / catch

try {
  Klass = new Phaser.Class(definition);
} catch (e) {
  if (e.message === 'initialize must be a function' && typeof definition.initialize !== 'function') {
    delete definition.initialize;
    Klass = new Phaser.Class(definition);
  } else { throw e; }
}

Prevention

Spell the key 'initialize' (American) and assign a function expression, not a name reference.
Run a lint rule that flags non-function values on the reserved 'initialize' key.
If no custom constructor is needed, omit initialize entirely.
When migrating ES6 classes, map `constructor` -> `initialize` explicitly, do not copy the class reference itself.

When it happens

Trigger: Passing `new Phaser.Class({ initialize: 'not a function' })`, `{ initialize: 42 }`, `{ initialize: { foo: 1 } }`, or `{ initialize: [function(){})] }`. Also from accidentally assigning a class reference or a config object to initialize instead of an actual function, or from a build step that serializes the definition and leaves initialize as a string.

Common situations: Refactoring a class and renaming the constructor without updating the initialize reference. Copy-pasting a class template and forgetting to replace the placeholder value. Transpiling/bundling where tree-shaking removes the function but leaves the key. Mixing ES6 class syntax (which has a real constructor) with Phaser.Class and assigning the wrong thing.

Related errors

cannot override final property '${k}', set Class.ignoreFinal

AI-assisted analysis of phaserjs/phaser@41be1e462b (2026-08-13). Data as JSON: /api/errors/1384bb87e2f93893. Report an issue: GitHub.