Error handling and testing
One 、 Error handling
One ）try-catch sentence
function atest() {
try {
// The code that could have an error
return 0;
} catch (error) {
// Error handler
console.log(error.message);
return 1;
} finally {
// It will be carried out anyway , The function finally returns 2
return 2;
}
}
Wrong type ：
1）Error: Error base class , Other error types are inherited by this class .
2）EvalError: Didn't put eval() When the function is called , The error will be sent .
3）RangeError: value out of range
4）ReferenceError: When the object is not found
5）SyntaxError: Pass in eval() Function syntax error
6）TypeError： The type of the variable does not meet the requirements
The rational use of try-catch:
Use try-catch It's best to deal with mistakes that we can't control . Suppose you're using a large JavaScript In the library
function , This function may throw some errors intentionally or unintentionally . Because we can't modify the source code of this library , Therefore, it is possible to reply to the letter
The number of calls is placed in try-catch In the sentence , In case something goes wrong , Or deal with them properly .
When you know your code is going to go wrong , Reuse try-catch The sentence is not suitable . for example , If
The arguments passed to the function are strings, not numbers , It will cause the function to go wrong , You should check the type of the parameter first , And then decide
How to do it . under these circumstances , Don't use try-catch sentence .
Two ） Throw an error
Use throw Operators can throw errors , There is no requirement for the thrown value
// It works
throw 111;
throw ''ss;
In case of throw When the operator , The code will stop executing immediately . Only if try-catch When the statement catches the value being thrown , generation
Code will continue to execute .
By using some built-in error type , More realistic simulation of browser errors . The constructor for each error type takes one parameter
Count , The actual error message .
throw new Error("An error");
When to throw and catch ：
Only those errors that you know exactly how to deal with should be caught . Catch the wrong
The goal is to avoid browsers processing them by default ; The purpose of throwing an error is to provide a message about the specific cause of the error .
3、 ... and ） Error events
Anything that doesn't pass try-catch Errors in processing will trigger window Object's error event , In any Web Browser , onerror Event handlers are not created event object ,
But it can take three parameters ： Error message 、 What's wrong URL And line number . To specify onerror Event handler , You must use the following DOM0 Level technology , It doesn't follow “DOM2 level
event ” Standard format .
window.onerror = function(msg, url, line) {
log(msg);
// Prevent browsers from reporting errors
return fales;
}
Two 、 test
One ） logging
// One for all browsers log
function log() {
try {
console.log.apply(console, arguments);
}
catch(e) {
try {
opera.postError.apply(opera, arguments);
}
catch (e) {
alert(Array.prototype.join.call(arguments, " "));
}
}
}
Two ） The test case
Three characteristics of a good test case ：
1） Reusability ： Multiple runs should produce the same result
2） simplicity ： Just focus on testing , Eliminate the impact of redundant code
3） independence ： Avoid one test result depending on another
3、 ... and ） test suite
The main purpose of the test suite is to aggregate all the individual tests of the code , Combine them into a unit , So they can run in batches , Provide a single resource that can be easily run over and over again .
1） Assertion ：
Make an assertion yourself ：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<style>
#results li.pass {
color: green;
}
#results li.fail {
color: red;
}
</style>
<body>
<ul id="results"></ul>
</body>
</html>
<script>
function assert(value, desc) {
var liElement = document.createElement("li");
liElement.className = value ? "pass" : "fail";
liElement.appendChild(document.createTextNode(desc));
document.getElementById("results").appendChild(liElement);
}
window.onload = function () {
assert(true, "Success");
assert(false, "Fail");
}
</script>
2） Implementation of test grouping ：
(function () {
var results;
this.assert = function assert(value, desc) {
var li = document.createElement("li");
li.className = value ? "pass" : "fail";
li.appendChild(document.createTextNode(desc));
results.appendChild(li);
if (!value) {
li.parentNode.parentNode.className = "fail";
}
return li;
}
this.test = function test(name, fn) {
results = document.getElementById("results");
results = assert(true, name).
appendChild(document.createElement("ul"));
fn();
}
})();
window.onload = function (ev) {
test("all true", function () {
assert(true, "111");
assert(true, "222");
assert(true, "333");
});
test("one false", function () {
assert(false, "failed");
assert(true, "success");
assert(true, "xxx");
});
}
3） Asynchronous test
(function () {
var queue = [], paused = false, results;
this.assert = function assert(value, desc) {
var li = document.createElement("li");
li.className = value ? "pass" : "fail";
li.appendChild(document.createTextNode(desc));
results.appendChild(li);
if (!value) {
li.parentNode.parentNode.className = "fail";
}
return li;
};
this.test = function test(name, fn) {
queue.push(function () {
results = document.getElementById("results");
results = assert(true, name).
appendChild(document.createElement("ul"));
fn();
});
runTest();
};
function runTest() {
if (!paused && queue.length) {
queue.shift()();
if (!paused) {
resume();
}
}
}
this.pause = function () {
paused = true;
};
this.resume = function resume() {
paused = false;
setTimeout(runTest, 1);
}
})();
window.onload = function (ev) {
test("First async test", function () {
pause();
setTimeout(function () {
assert(true, "First async completed!");
resume();
}, 1000);
});
test("Second async test", function () {
pause();
setTimeout(function () {
assert(true, "Second async test completed!");
resume();
}, 1000);
});
}
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