es6.md

ES6 and strict mode

ES6 modules are implicitly in strict mode.

See the spec

10.2.1 Strict Mode Code

    * Module code is always strict mode code.

ES6 default parameters

In ES6 functions can have default parameters:

function add(a, b=1) {
    return a + b;
}

add(1);    // 2
add(1, 2); // 3

Beware that default parameters are:

• evaluated at call time (a new object is created each time the function is called, unlike Python)
• default objects do not "traverse" properties, the entire object is "defaulted" (see example)

// accept 'first' and 'second' options
function f(options={ second=42 }) {
    options.first //...
    options.second //...
}

f();            // first=undefined, second=42
f({first: 41}); // first=41,        second=undefined

See MDN

Promises can "swallow" errors

If an error is thrown during the promise constructor or in one of the .then callbacks (onFulfilled/onRejected), the error is automatically caught and the promise rejected.

This is a good thing, however it can be perturbing when debugging. If no onRejected callback is attached the error will be "swallowed" and the failure silent.

For example:

const promise = new Promise(function() {
    throw new Error('boom');
});

// 'promise' is rejected, no 'onRejected' callback is attached so the failure is silent

Similarly:

const promise = new Promise(function(resolve, reject) {
    resolve(42);
});

const promise2 = promise.then(function(value) {
    throw new Error('boom');
});

// 'promise2' is now rejected, no onRejected callback so the failure is silent

The RSVP library helps solving this problem by proposing an error handler

RSVP.on('error', function(error) { // ... });

See JavaScript Promises - html5rocks

Rejections happen [...] implicitly if an error is thrown in the constructor callback.

[...] errors are automatically caught and become rejections.

The same goes for errors thrown in "then" callbacks.

See section 7. ii - Promises/A+ spec

If either onFulfilled or onRejected throws an exception e, promise2 must be rejected with e as the reason.

See also an interesting discussion on the v8 repository about this issue.