Auf dieser Seite sind verschiedene JavaScript Snippets zu allgemeinen Themen.

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01 · Abschnitt

Prüfen, ob Variable null oder undefined ist

JavaScript 
let myVar = null;

if (myVar === undefined || myVar === null) {
    console.log(`Variable is either undefined or null.`);
} else {
    console.log(`Variable is defined and not null.`);
}
Output 
Variable is either undefined or null.
02 · Abschnitt

Prüfen, ob Variable eine Funktion ist

JavaScript 
let myFunction = function() {
    console.log("Hello world");
};

if (typeof myFunction === "function") {
    console.log("Variable is of function type.");
} else {
    console.log("Variable is not of function type.");
}
Output 
Variable is of function type.
03 · Abschnitt

Queue-Struktur in JavaScript

JavaScript 
class Queue {

    constructor() {
        this.items = [];
    }

    enqueue(element) {
        this.items.push(element);
    }

    dequeue() {
        if (this.isEmpty()) {
            return "Underflow";
        }
        return this.items.shift();
    }

    front() {
        if (this.isEmpty()) {
            return "Queue is empty";
        }
        return this.items[0];
    }

    isEmpty() {
        return this.items.length === 0;
    }

    size() {
        return this.items.length;
    }

    print() {
        console.log(this.items);
    }

}

const queue = new Queue();

queue.enqueue(1);
queue.enqueue(2);
queue.enqueue(3);

console.log("Queue elements");
queue.print();

console.log("Front element:", queue.front());
console.log("Dequeued element:", queue.dequeue());
console.log("Queue size:", queue.size());
console.log("Is empty:", queue.isEmpty());
Output 
Queue elements
[ 1, 2, 3 ]
Front element: 1
Dequeued element: 1
Queue size: 2
Is empty: false
04 · Abschnitt

Prüfen, ob Variable eine Funktion ist

JavaScript 
let myFunction = function() {
    console.log("Hello world");
};

if (typeof myFunction === "function") {
    console.log("Variable is of function type.");
} else {
    console.log("Variable is not of function type.");
}
Output 
Variable is of function type.
05 · Abschnitt

Stack-Struktur in JavaScript

JavaScript 
class Stack {

    constructor() {
        this.items = [];
    }

    push(element) {
        this.items.push(element);
    }

    pop() {
        if (this.isEmpty()) {
            return "Underflow";
        }
        return this.items.pop();
    }

    peek() {
        return this.items[this.items.length - 1];
    }

    isEmpty() {
        return this.items.length === 0;
    }

    size() {
        return this.items.length;
    }

    print() {
        console.log(this.items);
    }
    
}

const stack = new Stack();

stack.push(1);
stack.push(2);
stack.push(3);

console.log("Stack elements");
stack.print();

console.log("Top element:", stack.peek());
console.log("Popped element:", stack.pop());
console.log("Stack size:", stack.size());
console.log("Is empty?", stack.isEmpty());
Output 
Stack elements
[ 1, 2, 3 ]
Top element: 3
Popped element: 3
Stack size: 2
Is empty? false
06 · Abschnitt

Zwei Variablen vertauschen

JavaScript 
let variableOne = "John";
let variableTwo = "Kate";

console.log("Before swapping");
console.log(`variableOne: ${variableOne}`);
console.log(`variableTwo: ${variableTwo}`);
console.log("=====");

// Swapping variables
let temp = variableOne;
variableOne = variableTwo;
variableTwo = temp;

console.log("After swapping");
console.log(`variableOne: ${variableOne}`);
console.log(`variableTwo: ${variableTwo}`);
Output 
Before swapping
variableOne: John
variableTwo: Kate
=====
After swapping
variableOne: Kate
variableTwo: John
07 · Abschnitt

Parameter in setTimeout übergeben

JavaScript 
function myFunction(parameter) {
    console.log("Parameter:", parameter);
}

let myParameter = "Hello world";
setTimeout(function() {
    myFunction(myParameter);
}, 1000);
Output 
Parameter: Hello world
08 · Abschnitt

Daten in Konsole eingeben

JavaScript 
const readline = require('readline').createInterface({
    input: process.stdin,
    output: process.stdout
})

readline.question('What is your name? ', name => {
    console.log(`Hello ${name}` )
    readline.close()
});
Output 
What is your name? Max
Hello Max
09 · Abschnitt

Umgebungsvariablen setzen

Bei der Ausführung von JS-Dateien in NodeJS kann man Umgebungsvariablen in das Script hineingeben.

JavaScript 
const id = process.env.USER_ID
const key = process.env.USER_KEY

console.log(id)
console.log(key)
JavaScript Verwendung 
USER_ID=1 USER_KEY=somekey node common_environment_variables.js
Output 
1
somekey
10 · Abschnitt

Fehler abfangen in try-catch

Der try-catch Block funktioniert wie eine Art Auffangschale. Vor allem der catch Block. Dieser fängt alle, im try Block, geworfenen Fehler auf und kann sie entsprechend behandeln.

JavaScript 
const myArray = [10, 23, 29, 34, 7, 39, 40];

try {
    
    if (myVar < 10) {
        throw new Error("My var is false");
    }

} catch (error) {
    console.log("ERROR IN CATCH:", error);
} finally {
    console.log("Done");
}
Output 
ERROR IN CATCH: ReferenceError: myVar is not defined
11 · Abschnitt

Typ der Variable ermitteln

Für die Ermittlung des Typs einer Variable, kann man sich eine Hilfs-Funktion schreiben.

JavaScript 
function getType(obj) {
    return Object.prototype.toString.call(obj).slice(8, -1);
}

console.log(getType(42));
console.log(getType("Hallo"));
console.log(getType(true));
console.log(getType([]));
console.log(getType({}));
console.log(getType(null));
console.log(getType(undefined));
console.log(getType(new Date()));
console.log(getType(/regex/));
console.log(getType(() => {}));
Output 
Number
String
Boolean
Array
Object
Null
Undefined
Date
RegExp
Function
/ Weiter

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