I am trying to understand how to use promises. I am having a little problem with pouchDB so I needed to go back and research how promises worked.
I have the following sample page I created with three promises and three status lines (two between promises and one after the promises).
Results as they appear on the page:
I think I understand why the straight document.writes happened first - they are quicker than a promise a JSON.parse.
I would have thought that #3 would have finished before #2 since it just has a straight document.write and #2 and 3 are promises and JSON.parse's. Also, why would #2 finish before #1?
Thanks,
Tom
I have the following sample page I created with three promises and three status lines (two between promises and one after the promises).
Code:
<!DOCTYPE html>
<html xmlns="[URL unfurl="true"]http://www.w3.org/1999/xhtml">[/URL]
<head>
<title></title>
</head>
<body>
<script>
var jsonPromiseBad = new Promise(function (resolve, reject) {
// JSON.parse throws an error if you feed it some
// invalid JSON, so this implicitly rejects:
resolve(JSON.parse("This ain't JSON<br><br>"));
});
var jsonPromiseGood = new Promise(function(resolve, reject) {
// JSON.parse throws an error if you feed it some
// invalid JSON, so this implicitly rejects:
resolve(JSON.parse('[1, 2, 3, 4 ]'));
});
var promiseFinished = new Promise(function (resolve, reject) {
// JSON.parse throws an error if you feed it some
// invalid JSON, so this implicitly rejects:
resolve('In promise "promiseFinished"!<br><br>');
});
jsonPromiseBad.then(function(data) {
// This never happens:
document.write("It worked! - Promise 1 - ", data + "<br><br>");
}).catch(function(err) {
// Instead, this happens:
document.write("It failed! - Promise 1 - ", err + "<br><br>");
});
document.write("Write between promises 1 and 2!<br><br><br>");
jsonPromiseGood.then(function (data) {
// This never happens:
document.write("It worked! - Promise 2 - ", data + "<br><br>");
}).catch(function (err) {
// Instead, this happens:
document.write("It failed! - Promise 2 - ", err + "<br><br>");
});
document.write("Write between promises 2 and 3!<br><br><br>");
promiseFinished.then(function (data) {
// This never happens:
document.write("It worked! - Promise 3 - ", data + "<br><br>");
}).catch(function (err) {
// Instead, this happens:
document.write("It failed! - Promise 3 - ", err + "<br><br>");
});
document.write("Write after promise 3!<br><br><br>");
</script>
</body>
</html>Results as they appear on the page:
Code:
Write between promises 1 and 2!
Write between promises 2 and 3!
Write after promise 3!
It worked! - Promise 2 - 1,2,3,4
It worked! - Promise 3 - In promise "promiseFinished"!
It failed! - Promise 1 - SyntaxError: Unexpected token T in JSON at position 0I think I understand why the straight document.writes happened first - they are quicker than a promise a JSON.parse.
I would have thought that #3 would have finished before #2 since it just has a straight document.write and #2 and 3 are promises and JSON.parse's. Also, why would #2 finish before #1?
Thanks,
Tom
