Can you have private methods in a function constructor in JavaScript?
Yes. Declare a function inside the constructor (not on this). It is only accessible inside the constructor and to methods that close over it. It is not accessible as a property on instances.
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More FAQs in Function Constructors and Closures in JavaScript
By declaring variables with let or var inside the constructor and creating methods on this that close over them. The variables are in the constructor's local scope and are not accessible as properties, only through the methods.
Methods created inside the constructor (on this) are per-instance, not shared on the prototype. Each instance gets its own copy, which uses more memory than prototype methods. ES6 classes with #private fields solve this.
ES6 classes with #private fields (ES2022). They provide true privacy without the memory cost of per-instance methods. Private fields are not accessible outside the class, and methods are shared on the prototype.
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