Question

When I put a comment above my variable or function with @private in it what is it actually doing? I have looked at the documentation but I’m still not sure.

goog.provide('myproject');

/** @private */
myproject.foo = "bar";

I can still access it when I open up chrome’s development tools (myproject.foo). And…

goog.require('myproject');

window.addEventListener('load', function() {
    //this works.
    document.body.textContent = myproject.foo;
});

The above code still set’s the bodies textContent to equal “bar”, even when compiled. So what does @private actually do?

Answer 1

The access control annotations @private, @protected, and @public are
directives for the Closure Compiler that help developers enforce the
desired level of visibility for properties and functions.

To emit warnings for access violations use the Compiler flag:

• –jscomp_warning=visibility

To emit errors for access violations use the Compiler flag:

• –jscomp_error=visibility

Access control annotations are enforced on a per file basis,
meaning that any properties annotated @private or @protected may be
accessed anywhere within the same file. Also note that the compiler removes
these annotations from the compiled code since they are not part of the
JavaScript language.

Example

file1.js

goog.provide('ns1');

/**
 * Global private variable.
 * @private
 */
ns1.global = 'foo';
alert('ns1.global = ' + ns1.global); // OK in same file.


/** @constructor */
ns1.Constructor = function() {
  /** @private */
  this.secret_ = ns1.global;
};

ns1.instance = new ns1.Constructor();
alert(ns1.instance.secret_); // No warning in same file.

file2.js

goog.provide('ns2');

goog.require('ns1');

alert('ns1.global = ' + ns1.global); // Not allowed.

ns2.instance2 = new ns1.Constructor();
alert(ns2.instance2.secret_); // Not allowed.

With the flag --jscomp_error=visibility set, Closure Compiler emits the
following errors.

ERROR - Access to private property global of ns1 not allowed here.
alert('ns1.global = ' + ns1.global);
                        ^
ERROR - Access to private property secret_ of ns1.Constructor not allowed here.
alert(ns2.instance2.secret_);
      ^

See Visibility (private and protected fields) in the Google JavaScript
Style Guide.