Uncaught SyntaxError: Unexpected token :

Updated answer

The nullish-coalescing operator was added in ECMAScript 11.

The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand.

let x = undefined; 
console.log(x ?? 'default');

x = null; 
console.log(x ?? 'default');

Original answer

Currently, there is no ?? operator in javascript.

You can use the ternary operator:

let x = undefined; 
console.log(x == undefined ? 'default' : x);

which will print 'default' if x is undefined, otherwise, it will print the value of x.

However, you've found the proposal for the nullish coalescing operator. This is currently a proposal in stage 3, but it looks like it will make it into a future version of the ECMAScript.

As mentioned in @Uncle's comment, that feature is only a proposal at this point and is not currently implemented in Javascript. To use it before it is implemented in modern browsers, you'll need to add this Babel plugin: @babel-plugin-proposal-nullish-coalescing-operator.