What is Node.js util.deprecate() ?

In Node.js, util.deprecate() wraps around a function in such a way that the function is marked deprecated. util.deprecate() throws a warning on STDERR, along with the supplied ERRCODE.

util.deprecate() is included in the util module. The util module can be accessed as follows:

const util = require('util');

Prototype

const funcName = util.deprecate(fn,msg,code)

Parameters

  • fn: The deprecated function

  • msg: The string that will be displayed on STDERR

  • code (optional): The deprecated code. See the full list of deprecated codes here.

Return value

util.deprecate() wraps and returns the function that is passed as the argument in such a way that it emits a warning onto the STDERR.

When util.deprecate() is called, it emits a DeprecationWarning using the ‘warning’ event. If the same code is provided for multiple calls, the warning will only be printed once.

Code

const util = require('util')
const test = util.deprecate(
    () => {
        console.log("DEPRICATED FUNC CALLED")
    },
    "test() is deprecated. Use updatedTest() instead",
    'ERR001'
)
test()

Here, we wrap the test() function using the util.deprecate() method. When test() is called, it emits a warning that is printed on STDERR and the deprecated code is executed.

If either the --no-deprecation or --no-warnings command-line flags are utilized, or if the process.noDeprecation property is set to true before the first deprecation warning, the util.deprecate() method does nothing.

If the --trace-deprecation or --trace-warnings command-line flags are set, or the process.traceDeprecation property is set to true, a warning and a stack trace are printed to STDERR when we invoke the deprecated function for the first time.

If the --throw-deprecation command-line flag is set, or the process.throwDeprecation property is set to true, then, at that point, an exception will be thrown when the deprecated function is called.

The --throw-deprecation command-line flag and process.throwDeprecation property outweigh --trace-deprecation and process.traceDeprecation.

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