How to Check If a Function Exists in JavaScript
When writing JavaScript code, it can be helpful to check if a function exists before using that function in future code. By using the "typeof" operator in an "if" statement, you can check if a function has been defined in the previous code and then determine how to handle that function. This is useful if you have been removing or rearranging sections of code and will prevent errors produced by calling a function that has been removed.
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Instructions
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Open the Web page document that contains the JavaScript code in your default text editor or a text-based HTML editor.
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Click to place your cursor on the line in the JavaScript code where you want to check if a function exists.
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Type "if(typeof functionName == 'function') {" then press "ENTER." Replace "functionName" with the name of your function (do not include parenthesis). This line of code opens the "if" statement and inserts the "typeof" operator. The function is then checked to see if it has been defined as a function in the previous code.
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Type "functionName;" then press "ENTER." Replace "functionName" with the name of your function. This line of code calls the function, if it is found to exist in the previous lines of code.
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Type "}" to close the statement.
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Click "File" and "Save" to save the Web page document.
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