The design of the Javascript language is not rigorous enough, and errors can occur in many places if you are not careful.
For example, consider the following situation.
Now, we need to determine whether a global object myObj exists. If it does not exist, declare it. The algorithm described in natural language is as follows:
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if (myObj does not exist){
declare myObj;
}
You might think that writing this code is easy. But in fact, the grammatical issues involved are far more complex than we imagine. Juriy Zaytsev pointed out that there are more than 50 ways to determine whether a Javascript object exists. Only if you are very clear about the implementation details of the Javascript language can you tell the difference between them.
The first way of writing
Intuitively, you might think you could write:
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if (!myObj) {
myObj = { };
}
However, when running this code, the browser will directly throw a ReferenceError error, causing the operation to be interrupted. What's wrong?
By the way, when the if statement determines whether myObj is empty, the variable does not exist yet, so an error is reported. Change it to the following and it will run correctly.
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if (!myObj) {
var myObj = { };
}
Why is there no error after adding a var? Could it be that in this case, when the if statement makes a judgment, does myObj already exist?
To answer this question, you must know how the Javascript interpreter works. The Javascript language is "parse first, run later". The variable declaration has already been completed during parsing, so the above code is actually equivalent to:
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var myObj;
if (!myObj) {
var myObj = { };
}
Therefore, when the if statement makes a judgment, myObj does already exist, so no error is reported. This is the "hoisting" effect of the var command. The Javascript interpreter only "promotes" variables defined by the var command, and does not work on variables that are directly assigned without using the var command. This is why an error will be reported if var is not added.
The second way of writing
In addition to the var command, there is another rewriting that can also get the correct results:
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if (!window.myObj) {
myObj = { };
}
Window is the top-level object of JavaScript, and all global variables are its properties. Therefore, determining whether myobj is empty is equivalent to determining whether the window object has the myobj attribute, so that ReferenceError errors that occur because myObj is not defined can be avoided. However, from the standardization of the code, it is best to add var to the second line:
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if (!window.myObj) {
var myObj = { };
}
Or written like this:
if (!window.myObj) {
window.myObj = { };
}
The third way of writing
The disadvantage of the above writing method is that in some running environments (such as V8, Rhino), window may not be a top-level object. So, consider rewriting it as:
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if (!this.myObj) {
this.myObj = { };
}
At the level of global variables, the this keyword always points to the top-level variable, so it can be independent of different running environments.
The fourth way of writing
However, the above way of writing is less readable, and the pointer of this is variable and error-prone, so we rewrite it further:
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var global = this;
if (!global.myObj) {
global.myObj = { };
}
It is much clearer to use the custom variable global to represent the top-level object.
The fifth way of writing
You can also use the typeof operator to determine whether myObj is defined.
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if (typeof myObj == "undefined") {
var myObj = { };
}
This is currently the most widely used method to determine whether a JavaScript object exists.
The sixth way of writing
Since the value of myObj is directly equal to undefined when it is defined but not assigned, the above writing method can be simplified:
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if (myObj == undefined) {
var myObj = { };
}
There are two things to note here. First, the var keyword in the second line cannot be missing, otherwise a ReferenceError will occur. Secondly, undefined cannot be added with single quotes or double quotes, because the data type of undefined is compared here, not "undefined "This string.
The seventh way of writing
The above writing method still holds true in the case of "exact comparison" (===):
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if (myObj === undefined) {
var myObj = { };
}
The eighth way of writing
According to the language design of JavaScript, undefined == null, so comparing whether myObj is equal to null can also get the correct result:
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if (myObj == null) {
var myObj = { };
}
However, although the running results are correct, from a semantic point of view, this judgment method is wrong and should be avoided. Because null refers to an empty object that has been assigned a value of null, that is, this object actually has a value, while undefined refers to an object that does not exist or has no value assigned. Therefore, only the "comparison operator" (==) can be used here. If the "exact comparison operator" (===) is used here, an error will occur.
The ninth way of writing
You can also use the in operator to determine whether myObj is an attribute of the top-level object:
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if (!('myObj' in window)) {
window.myObj = { };
}
The tenth way of writing
Finally, use the hasOwnProperty method to determine whether myObj is a property of the top-level object:
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if (!this.hasOwnProperty('myObj')) {
this.myObj = { };
}
Summarize
1. If you only determine whether the object exists, it is recommended to use the fifth writing method.
2. If in addition to whether the object exists, you also need to determine whether the object has a null value, it is recommended to use the first way of writing.
3. Unless there are special circumstances, all variables should be declared using the var command.
4. In order to be cross-platform, it is recommended to avoid using window to represent top-level objects.
5. In Javascript language, null and undefined are easily confused. In cases where both may be involved, it is recommended to use the "exact comparison" operator (===).