Node.js displays "undefined" on the console

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undefined
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node also prints undefined because IN GENERAL, it displays the return value of each command and console.log doesn't return anything., GitLab launches Collective on Stack Overflow ,Stack Overflow en español, Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers

As pointed out by others, javascript function will always return undefined if you do not specify any return value. You can just ignore it. It's not going to cause any harm. But if it's annoying you too much then you can turn it off in repl. Repl has this property ignoreUndefined which is set to false by default. You can set it to true. Try this:

module.exports.repl.ignoreUndefined = true;
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The global undefined property represents the primitive value undefined. It is one of JavaScript's primitive types. , undefined is a property of the global object. That is, it is a variable in global scope. The initial value of undefined is the primitive value undefined. ,ReferenceError: reference to undefined property "x",Transitioning to strict mode

undefined
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label <string> The display label for the counter. Default: 'default'.,label <string> Default: 'default',console.count([label]),Resets the internal counter specific to label.

Example using the global console:

console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:

const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err

The Console class can be used to create a simple logger with configurable output streams and can be accessed using either require('console').Console or console.Console (or their destructured counterparts):

const {
   Console
} = require('console');
const {
   Console
} = console;

Creates a new Console with one or two writable stream instances. stdout is a writable stream to print log or info output. stderr is used for warning or error output. If stderr is not provided, stdout is used for stderr.

const output = fs.createWriteStream('./stdout.log');
const errorOutput = fs.createWriteStream('./stderr.log');
// Custom simple logger
const logger = new Console({
   stdout: output,
   stderr: errorOutput
});
// use it like console
const count = 5;
logger.log('count: %d', count);
// In stdout.log: count 5

The global console is a special Console whose output is sent to process.stdout and process.stderr. It is equivalent to calling:

new Console({
   stdout: process.stdout,
   stderr: process.stderr
});

If value is truthy, nothing happens.

console.assert(true, 'does nothing');

console.assert(false, 'Whoops %s work', 'didn\'t');
// Assertion failed: Whoops didn't work

console.assert();
// Assertion failed

Maintains an internal counter specific to label and outputs to stdout the number of times console.count() has been called with the given label.

> console.count()
default: 1
undefined
   >
   console.count('default')
default: 2
undefined
   >
   console.count('abc')
abc: 1
undefined
   >
   console.count('xyz')
xyz: 1
undefined
   >
   console.count('abc')
abc: 2
undefined
   >
   console.count()
default: 3
undefined
   >

Resets the internal counter specific to label.

> console.count('abc');
abc: 1
undefined
   >
   console.countReset('abc');
undefined
   >
   console.count('abc');
abc: 1
undefined
   >

Prints to stderr with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr

Prints to stdout with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout

Try to construct a table with the columns of the properties of tabularData (or use properties) and rows of tabularData and log it. Falls back to just logging the argument if it can’t be parsed as tabular.

// These can't be parsed as tabular data
console.table(Symbol());
// Symbol()

console.table(undefined);
// undefined

console.table([{
   a: 1,
   b: 'Y'
}, {
   a: 'Z',
   b: 2
}]);
// ┌─────────┬─────┬─────┐
// │ (index) │  a  │  b  │
// ├─────────┼─────┼─────┤
// │    0    │  1  │ 'Y' │
// │    1    │ 'Z' │  2  │
// └─────────┴─────┴─────┘

console.table([{
   a: 1,
   b: 'Y'
}, {
   a: 'Z',
   b: 2
}], ['a']);
// ┌─────────┬─────┐
// │ (index) │  a  │
// ├─────────┼─────┤
// │    0    │  1  │
// │    1    │ 'Z' │
// └─────────┴─────┘

Stops a timer that was previously started by calling console.time() and prints the result to stdout:

console.time('100-elements');
for (let i = 0; i < 100; i++) {}
console.timeEnd('100-elements');
// prints 100-elements: 225.438ms

For a timer that was previously started by calling console.time(), prints the elapsed time and other data arguments to stdout:

console.time('process');
const value = expensiveProcess1(); // Returns 42
console.timeLog('process', value);
// Prints "process: 365.227ms 42".
doExpensiveProcess2(value);
console.timeEnd('process');

Prints to stderr the string 'Trace: ', followed by the util.format() formatted message and stack trace to the current position in the code.

console.trace('Show me');
// Prints: (stack trace will vary based on where trace is called)
// Trace: Show me
// at repl:2:9
// at REPLServer.defaultEval (repl.js:248:27)
// at bound (domain.js:287:14)
// at REPLServer.runBound [as eval] (domain.js:300:12)
// at REPLServer.<anonymous> (repl.js:412:12)
   // at emitOne (events.js:82:20)
   // at REPLServer.emit (events.js:169:7)
   // at REPLServer.Interface._onLine (readline.js:210:10)
   // at REPLServer.Interface._line (readline.js:549:8)
   // at REPLServer.Interface._ttyWrite (readline.js:826:14)

This method does not display anything unless used in the inspector. The console.profile() method starts a JavaScript CPU profile with an optional label until console.profileEnd() is called. The profile is then added to the Profile panel of the inspector.

console.profile('MyLabel');
// Some code
console.profileEnd('MyLabel');
// Adds the profile 'MyLabel' to the Profiles panel of the inspector.
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On execution of JavaScript, I get an undefined message along with successful execution of the expression. ,Recently I installed node.js on my Windows 7 machine. ,Just write "hello world"; and hit enter... it will return "hello world" instead of undefined, thus no undefined is displayed. console.log returns undefined and also logs arguments to console so you get multiple messages.,Callback - In JavaScript, higher order functions could be passed as parameters in functions. Since JavaSCript is a single threaded, only one operations happens at a time, each operation thats going to happen is queued in single thread. This way, passed functions(as parameter) could be executed when rest of the parent functions operation(async) is completed and script can continue executing while waiting for results.

Move the following line:

res.render("index.html", content);

into the hasNextPage callback:

functions.getRecentPost = function(req, res, next) {

   //.........

   es.search(query, function(err, data) {

      //.........

      hasNextPage(_page, content.posts.length, function(data) {

         content.hasNext = data;
         res.render("index.html", content);

      });
   });
};

If you expect getRecentPost to return something then you need to add a callback to it as well so that you can use it's return value. For example, if you expect to be doing this:

functions.getRecentPost = function(req, res, next) {

   //......

   return content;
}

doSomething(functions.getRecentPost(x, y, z));

It won't work because the final value of content will be retrieved asynchronously. Instead you need to do this:

functions.getRecentPost = function(req, res, next, callback) {

   //.........

   hasNextPage(_page, content.posts.length, function(data) {

      content.hasNext = data;
      res.render("index.html", content);

      callback(content);

   });
};

functions.getRecentPost(x, y, z, function(content) {
   doSomething(content);
})

You cannot return data asynchronously. You need to change your code (and your thinking) from writing stuff like this:

asyncFunction(function(data) {
   foo = data;
});

doSomething(foo);

into this:

asyncFunction(function(data) {
   doSomething(data);
});

Regular imperative code looks like this:

function fN() {
   x = fA();
   y = fB(x);
   z = fC(y);
   return fD(fE(z));
}

asynchronous code looks like this:

function fN(callback) {
   fA(function(x) {
      fB(x, function(y) {
         fC(y, function(z) {
            fE(z, function(zz) {
               fD(zz, function(zzz) {
                  callback(zzz);
               });
            });
         });
      });
   });
}
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To begin REPL, simply type node without providing any Node application file, like so:,You can access this application from a browser no differently than if you had typed the text into a file and run it using Node.,Running the application with Node brings up the REPL prompt, where we can then access the modules:,You can use the variable in REPL, just as you would in a Node application:

To begin REPL, simply type node without providing any Node application file, like so:

$ node

REPL is very simple to use. Just start typing in your JavaScript, like you’d add it to a file:

> a = 2;
2

The tool prints out the result of whatever expression you just typed. In this session excerpt, the value of the expression is 2. In the following, the expression result is an array with three elements:

> b = ['a', 'b', 'c'];
['a', 'b', 'c']

To access the last expression, use the underscore/underline special variable (_). In the following, a is set to 2, and the resulting expression is incremented by 1, and then 1 again:

> a = 2;
2
   >
   ++_;
3
   >
   ++_;
4

You can even access properties or call methods on the underscored expression:

> ['apple', 'orange', 'lime']
['apple', 'orange', 'lime'] >
_.length
3
   >
   3 + 4
7
   >
   _.toString();
'7'

You can use the var keyword with REPL in order to access an expression or value at a later time, but you might get an unexpected result. For instance, consider the following line in REPL:

var a = 2;

Consider the following instead, which is what’s happening, more or less, under the hood in REPL:

console.log(eval('a = 2'));
console.log(eval('var a = 2'));

Typing the preceding line into a file and running that file using Node returns:

2
undefined

You can use the variable in REPL, just as you would in a Node application:

>
var a = 2;
undefined
   >
   a++;
2
   >
   a++;
3
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How to check if a variable exists or defined in JavaScript,How to check whether a value is numeric or not in jQuery,In JavaScript if a variable has been declared, but has not been assigned a value, is automatically assigned the value undefined. Therefore, if you try to display the value of such variable, the word "undefined" will be displayed. Whereas, the null is a special assignment value, which can be assigned to a variable as a representation of no value.,In simple words you can say a null value means no value or absence of a value, and undefined means a variable that has been declared but no yet assigned a value.

<script>
var firstName;
var lastName = null;
// Try to get non existing DOM element
var comment = document.getElementById('comment');

console.log(firstName); // Print: undefined
console.log(lastName);  // Print: null
console.log(comment);   // Print: null

console.log(typeof firstName); // Print: undefined
console.log(typeof lastName);  // Print: object
console.log(typeof comment);   // Print: object

console.log(null == undefined)  // Print: true    
console.log(null === undefined) // Print: false

/* Since null == undefined is true, the following statements will catch both null and undefined */
if(firstName == null){
    alert('Variable "firstName" is undefined.');
}    
if(lastName == null){
    alert('Variable "lastName" is null.');
}

/* Since null === undefined is false, the following statements will catch only null or undefined  */
if(typeof comment === 'undefined') {
    alert('Variable "comment" is undefined.');
} else if(comment === null){
    alert('Variable "comment" is null.');
}
</script>

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