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I have the following:
new Promise(resolve => setTimeout(resolve, 2000))
.then(() => console.log("after 2 seconds"));
new Promise(resolve => setTimeout(resolve, 3000))
.then(console.log("before 3 seconds (instantly)"));
which produces the following output:
> node index.js
before 3 seconds (instantly)
after 2 seconds
Promise.then() expects a onFulfilled function, but I passed in console.log("before 2 seconds (instantly)"), which is not a function. Two-part question:
console.log("before 2 seconds (instantly)") get executed right away (or at all)?
772
To pass parameters to a promise function with JavaScript, we can create a function that takes the parameters we want to returns the promise. to create the f function that takes the username and password . In it, we return a promise created by the Promise constructor.
The code inside the Promise constructor runs when the promise is created and it runs synchronously which surprises some people. So even without then() everything still runs.
all() The Promise. all() method takes an iterable of promises as an input, and returns a single Promise that resolves to an array of the results of the input promises. This returned promise will fulfill when all of the input's promises have fulfilled, or if the input iterable contains no promises.
The code
console.log("before 3 seconds (instantly)")
is an expression, specifically a function call expression. Wherever that appears, it means the same thing, including an appearance as an argument to the .then() method of a Promise. As in any other similar language, an expression used in a function call is evaluated before the function call, so
.then(console.log("before 3 seconds (instantly)"))
results in the console.log() function being called first, with the return value then passed to .then(). That's why you see the message in the console immediately.
Passing undefined to .then() is allowed, and since that's what console.log() returns, there's no error raised.
If you want that console.log() to happen when the Promise is fulfilled, you'd wrap it in a function:
.then(function() { console.log("after 3 seconds"); })
Why is it possible to pass in a non-function parameter to Promise.then() without causing an error?
Yes. All non-function arguments should be ignored. See below.
Why does console.log("before 2 seconds (instantly)") get executed right away (or at all)?
Because in JS the arguments to functions calls are evaluated instantly (applicative order).
Why didn't the second Promise raise an exception when I didn't pass in a function?
Because console.log returns undefined and .then() with no arguments is legal (because both handlers are optional). In your example console.log() returns undefined so it is like calling .then() with no arguments.
But even if it was called with some arguments that are not functions, they would still get ignored. For example even in this example the 'ok' would still get to the console.log at the end, which may be surprising:
Promise.resolve('ok')
.then()
.then(false)
.then(null)
.then(1)
.then('x')
.then([1, 2, 3])
.then({a: 1, b: 2})
.then(console.log);
See the Promises/A+ specification, section 2.2.1 that describe the arguments to the .then() method:
2.2.1 Both onFulfilled and onRejected are optional arguments:
- If onFulfilled is not a function, it must be ignored.
- If onRejected is not a function, it must be ignored.
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