Скачать или смотреть How to Check if a Variable is Defined in JavaScript

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Summary: Learn various methods to check if a variable is defined in JavaScript, including using `typeof`, `if` statements, and try-catch blocks to handle undefined variables effectively.
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In JavaScript, ensuring that a variable is defined before using it is crucial to avoid runtime errors. Here are several methods to check if a variable is defined.

Using typeof

The typeof operator is a safe way to check if a variable is defined because it does not throw an error if the variable is not defined.

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Using if Statements

You can use if statements combined with the typeof operator to check for both undefined and null.

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Checking for null and undefined

To handle both null and undefined, you can use strict equality checks:

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Using try-catch

If you want to catch errors for variables that might not be defined in any scope, a try-catch block can be used.

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Combining with hasOwnProperty

For object properties, you can use hasOwnProperty to check if a property exists on an object.

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Using Optional Chaining (ES2020)

Optional chaining allows you to check if deeply nested properties are defined without throwing an error.

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Conclusion

Each of these methods offers a way to check if a variable is defined in JavaScript, suitable for different scenarios. Using typeof is generally the safest and most straightforward method for general cases, while try-catch and optional chaining are useful for handling more complex situations.

By employing these techniques, you can write more robust and error-resistant JavaScript code.