Why Export Default Syntax is Not Allowed With Const

const keyword is used to declare variables in JavaScript.

const a = 10;

We can declare multiple constants at the same time.

const a = 10,
  b = 20;

We can export constants using export keyword.

export const a = 10,
  b = 20;

But, JavaScript does not support export with default along with const. For example, below code is invalid.

export default const a = 10, b = 20;

It is because we cannot identify which is the default out of a and b at the time of import.

Therefore, in case of constants we need to do the declaration and exporting in two different steps.

const a = 10,
  b = 20;

export default a;

Due to this reason, if you try to export a function expression as default, it will not work. Below code does NOT work.

export default const sum = (a, b) => {
    return a + b;
}

If you do not like the export default to be on separate line, above code can be rewritten as:

export default function sum(a, b) {
  return a + b;
}

Above code is valid because export default can be used along with expressions, functions and classes.