Are you annoyed by the following error and want to put check to see if a variable exists before accessing them?
ReferenceError: x is not defined
In this tutorial, we will discuss the same.
Checking if a variable is defined
You can use the following condition to check if x
is defined:
if (typeof x !== "undefined") {
console.log(x)
}
The typeof operator
returns the type of the variable and if it is not defined, or has not been initialized, returns "undefined"
.
It can also be checked if the variable is initialized or not. In the below code, the control will not go inside the if
block.
let x
if (typeof x !== "undefined") {
console.log(x)
}
You will see the value of x
logged into the console, if you execute the following code:
let x = 1
if (typeof x !== "undefined") {
console.log(x)
}
Checking if an object property is defined
If you need to check if a property exists within an object
then you can use the prototype method hasOwnProperty
const person = {
name: "John",
}
if (person.hasOwnProperty("age")) {
console.log(person.age)
}
The above code will not log anything to the console.
The same can be used in the case of the window object. The below code will log 10
to the console.
window.y = 10
if (window.hasOwnProperty("y")) {
console.log(window.y)
}
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