How to implement object internal slot property

Question

I recognize that some Javascript Objects has some internal slot property surrounding with double square bracket [[ ]] and can only be accessed by a method.

For example:

var str = new String('example');

When inspecting variable str , I see the property [[PrimitiveValue]] has the example value, but I cannot access this property because it's internal. The only way to access it by the method toString() only.

My question is: how can I implement the custom object with that behavior?, having an internal property with [[ ]] surrounding and a method to access that property.

Answer 1

From this shim on Github :

var SLOT = require('internal-slot');
var assert = require('assert');

var o = {};

assert.throws(function () { SLOT.assert(o, 'foo'); });

assert.equal(SLOT.has(o, 'foo'), false);
assert.equal(SLOT.get(o, 'foo'), undefined);

SLOT.set(o, 'foo', 42);

assert.equal(SLOT.has(o, 'foo'), true);
assert.equal(SLOT.get(o, 'foo'), 42);

assert.doesNotThrow(function () { SLOT.assert(o, 'foo'); });