promises-async-values.md

Async-sync correspondence

Just like typical expressions are evaluated at time of assignment, promise expressions are also evaluated eagerly.

Promise values act like regular values. You can reuse regular variable values later:

var x = someExpression()

// some other method or function, later:
var y = x + 1

just like you can reuse promises later:

var x = someExpression()

// some other method or function, later:
var y = x.then(x => x + 1)

You can use regular variables multiple times. If you do that, it will not cause recomputation of the expression:

var x = someExpression()
var y = x + 1
var z = x + 2

Similarly you can use promises multiple times, and it will not cause re-execution of the operation:

var x = someExpression()
var y = x.then(x => x + 1)
var z = x.then(x => x + 2)

Just like you wouldn't expect to be able to control the evaluation time of a value, similarly you can't control the evaluation of a promise. You need to wrap both in a function if you want to evaluate them later:

var x = () => someExpression()

Where analogy breaks down

In sync code, to handle errors we use try-catch. We must wrap the block in try-catch before evaluating:

try {
  var x = someExpression()
} catch (e) {
  // ...
}

Promises have their async correspondence to try-catch:

var x = someExpression()

x.catch(e => {
});

We don't expect to be able to do this for sync value expressions:

var x = someExpression()

try {
  x
} catch (e) {
}

Similarly, since promises evaluate asynchronously, its reasonable to expect that the catch handler is added at the same event loop tick, and to enforce that via throwing if such a handler is not attached by the next tick of the event loop.