Default function parameters allow named parameters to be initialized with default values if no value or undefined is passed. This ES6 feature makes functions more robust, readable, and less error-prone.

Why Use Default Parameters?

1. Cleaner code — No more verbose typeof checks
2. Better API design — Clear expectations for optional arguments
3. Fewer bugs — Prevent unexpected undefined values from causing NaN or errors
4. Self-documenting — The default value serves as inline documentation

Syntax

function [name]([param1[ = defaultValue1 ][, ..., paramN[ = defaultValueN ]]]) {
   statements
}

Description

In JavaScript, function parameters default to undefined. However, it’s often useful to set a different default value. This is where default parameters can help.

In the past, the general strategy for setting defaults was to test parameter values in the function body and assign a value if they are undefined.

In the following example, if no value is provided for b when multiply is called, b’s value would be undefined when evaluating a * b and multiply would return NaN.

function multiply(a, b) {
  return a * b;
}

multiply(5, 2); // 10
multiply(5);    // NaN !

To guard against this, something like the second line would be used, where b is set to 1 if multiply is called with only one argument:

function multiply(a, b) {
  b = (typeof b !== 'undefined') ?  b : 1;
  return a * b;
}

multiply(5, 2); // 10
multiply(5);    // 5

With default parameters in ES2015, checks in the function body are no longer necessary. Now, you can assign 1 as the default value for b in the function head:

function multiply(a, b = 1) {
  return a * b;
}

multiply(5, 2); // 10
multiply(5);    // 5

References

• Default parameters - MDN