How to check if a variable exists or is defined in JavaScript

In some cases, your JavaScript code may need to depend on a particular variable. If you want to check if a variable has been initialized or defined (i.e. if a variable has been declared and received a value), you can use the typeof operator.

The typeof operator is useful for checking the data type of a variable. Here is a list of the values returned by the typeof operator:

• “Number” – the variable is a number.
• “String” – the variable is a string.
• “Boolean” – the variable is a boolean.
• “Object” – the variable is an object.
• “Null“ – the variable is null.
• “Undefined” – the variable is undefined.

So in this case, to check if a variable exists or is defined, use the typeof operator and check if the returned value is “undefined”.
 
 

How to check if a variable exists or is defined in JavaScript

var nbr = 10;

if (typeof nbr == 'number') {
    document.write(nbr + " is a number <br/>");
}

if (typeof nbr == 'undefined') {
    document.write("The variable nbr does not exist <br/>");
} else {
    document.write("The variable nbr exists<br/>");
}

if (typeof str == 'undefined') {
    document.write("The variable str does not exist <br/>");
} else {
    document.write("The variable str exists<br/>");
}

Output:

10 is a number
The variable nbr exists
The variable str does not exist

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