Check if a Parameter is provided to Function in JavaScript

# Check if a Parameter is provided to Function in JavaScript

To check if a parameter is provided to a function, use the strict inequality (!==) operator to compare the parameter to undefined.

If the comparison returns true, then the parameter was provided to the function.

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function getParam(param) {
  if (param !== undefined) {
    console.log('The parameter was provided to the function');
    return param;
  }
}

console.log(getParam(5)); // 👉️ 5
console.log(getParam()); // 👉️ undefined

We used the strict inequality (!==) operator to check if the value of the parameter is not equal to undefined.

If the condition passes, then the parameter is provided to the function.

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function getParam(param) {
  if (param !== undefined) {
    console.log('The parameter was provided');
    return param;
  } else {
    // 👇️ this runs
    console.log('The parameter was not provided');
  }
}

console.log(getParam(undefined)); // 👉️ undefined

I can't think of a good use case to do this, so I don't see this as a drawback.

If any other value is passed to the function, the if block runs.

# Check if a Parameter is provided to Function using typeof

Alternatively, you can use the typeof operator.

If the condition is met, the parameter is provided to the function.

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function getParam(param) {
  if (typeof param !== 'undefined') {
    return param;
  }
}

console.log(getParam(5)); // 👉️ 5
console.log(getParam()); // 👉️ undefined

We used the typeof operator to get a string that indicates the type of a value.

Here are some examples of using the typeof operator.

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console.log(typeof undefined); // 👉️ "undefined"
console.log(typeof (() => {})); // 👉️ "function"
console.log(typeof function () {}); // 👉️ "function"
console.log(typeof null); // 👉️ "object"
console.log(typeof []); // 👉️ "object"
console.log(typeof {}); // 👉️ "object"
console.log(typeof ''); // 👉️ "string"
console.log(typeof 0); // 👉️ "number"

When the typeof operator is used on an undefined value, it returns the string "undefined", which is exactly what we check for in our if statement.

# Assigning default values for function parameters

You can assign a default value for the parameter if there is a value that suits your use case.

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function getParam(param = 'hello') {
  return param;
}

console.log(getParam()); // 👉️ hello

console.log(getParam('world')); // 👉️ world

We used a default value of the string hello for when the function is invoked without a parameter.

The default value is also used if the function gets passed a value of undefined.

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function getParam(param = 'hello') {
  return param;
}

console.log(getParam(undefined)); // 👉️ hello

console.log(getParam('world')); // 👉️ world

If the function gets called with a value for the parameter, the supplied value is used.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• How to Check if a Function is Async in JavaScript
• How to Check if a Function is Defined in JavaScript
• How to Check if a Function returns True in JavaScript