util.deprecate does not work as expected

[hiiwave]

• Version: v7.10.1
• Platform: Linux linsim-4 4.10.0-27-generic README still points to nodejs IRC room #30~16.04.2-Ubuntu SMP Thu Jun 29 16:07:46 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
• Subsystem:

According to the document, by calling util.deprecate(function, string),
it should run the function before printing the string on stderr.
However, I find that the function does NOT run when I call util.deprecate.

To reproduce it, use the sample code in the document

// test.js
const util = require('util');

exports.puts = util.deprecate(function() {
  for (let i = 0, len = arguments.length; i < len; ++i) {
    process.stdout.write(arguments[i] + '\n');
  }
}, 'util.puts: Use console.log instead');

And the output is nothing:

$ node test.js
$ 

Another example is as follows:

const util = require('util');
console.log(util.deprecate);
var f = function() {
    console.log("F");
};
util.deprecate(f, "DEPRECATED");
console.log("END");

And the output does not show F as expected:

$ node test.js
[Function: deprecate]
END
$ 

Do I have any misunderstanding or is there any tricky issue upon this function? Thanks a lot.

[vsemozhetbyt]

util.deprecate() returns the function wrapped. You need to call the returned function to get the warning:

const util = require('util');

const f = util.deprecate(function() { console.log("F"); }, "DEPRECATED");

f();

F
(node:1176) DeprecationWarning: DEPRECATED

[hiiwave]

Oh, that's my carelessness. Thank you very much for your help.