This TypeError is thrown in JavaScript when you attempt to call a value as a function, but that value is not a function. The 'X' in the message is the name or expression that was called. It typically means a property that should be a function is undefined, null, or a different type.
Why it happens
- •Calling a method that does not exist on an object (the property is undefined).
- •A variable was reassigned from a function to a non-function value before the call.
- •Forgetting to require or import a module and calling a property on the undefined result.
- •Accidentally overwriting a built-in method such as Array.prototype.map with a non-function value.
Examples
Code that triggers the error
const config = {
port: 3000,
};
config.start();Error output
TypeError: config.start is not a function
How to fix it
Log the value before calling it to confirm it is actually a function: console.log(typeof myValue). Check for typos in method names. If the method comes from an external source (API response, configuration), validate its type before calling it.
Fixed code
const config = {
port: 3000,
start() {
console.log(`Server starting on port ${this.port}`);
},
};
config.start(); // Server starting on port 3000Related errors
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