In the source code of vue,vue/src/shared/There are some methods stored in the file. The author used this method to judge the type, but it was not used directly, but saved in a variable separately:
const _toStr =
So why do you do this?
Let’s talk about the type of judgment first. As we all know, typeof cannot correctly judge Null when judging an object, and cannot recognize Array, but it is OK when judging the basic type. So Youda also wrote:
export function isPrimitive (value: any): boolean %checks {
return (
typeof value === 'string' ||
typeof value === 'number' ||
// $flow-disable-line
typeof value === 'symbol' ||
typeof value === 'boolean'
)
}
The judgment Object also makes a distinction, isObject and isPlainObject:
export function isObject (obj: mixed): boolean %checks {
return obj !== null && typeof obj === 'object'
}
export function isPlainObject (obj: any): boolean {
return _toString.call(obj) === '[object Object]'
}
When it comes to judging complex types, we usually useOr instanceof. If it is the former, it will return a string similar to '[object Object]'. The latter will determine whether there is a constructor on the prototype chain of an object.
There are some differences between the two.(1) and(Number(1))When the return is"[object Number]"，That is, it does not distinguish between primitive and complex types. visible,Not as useful as many tutorials say.
(1) "[object Number]" (Number(1)) "[object Number]"
If you want to use it, you need to take out the original type separately and then judge the complex type like Youda. When you reach this point, Youda wrote the line const _toStr. This is because toString is too easy to be rewritten. If toString is rewritten by others, it will affect the part involved in the code, so save it to prevent this from happening.
Supplementary: Principles of the method
Preface
When we judge the built-in type of an object, we can generally use the following methods:
var arr = []; ((arr)) //"[object Array]"
So, what is the principle of this method?
ECMAScript 3
When the toString method is called, the following steps will be performed:
1. Get the value of the [[Class]] property of this object.
2. Calculate the three strings "[object", the operation result of the first step Result(1), and the new string after "]".
3. Return the operation result of the second step Result(2).
[[Class]] is an internal property, and all objects (native and host objects) have this property. In the specification, [[Class]] is defined in this way
[[Class]] A string value indicating the type of the object.
Then gave an explanation:
The values of the [[Class]] attribute of all built-in objects are defined by this specification. The values of the [[Class]] attribute of all host objects can be any value, or even the values of the [[Class]] attribute used by the built-in object. The values of the [[Class]] attribute can be used to determine which built-in type a native object belongs to. It should be noted that, except through methods, this specification does not provide any other way to allow the program to access the value of the attribute.
ECMAScript 5
When the toString method is called, the following steps will be performed
If the value of this is undefined, "[object Undefined]" is returned.
If the value of this is null, "[object Null]" is returned.
Let O be the result of calling ToObject(this).
Make class the value of O's internal property [[Class]].
Returns the new string after three strings "[object", class, and "]".
In ES5, the explanation of the [[Class]] attribute is more detailed:
The values of the [[Class]] attributes of all built-in objects are defined by this specification. The values of the [[Class]] attributes of all host objects can be any string except "Arguments", "Array", "Boolean", "Date", "Error", "Function", "JSON", "Math", "Number", "Object", "RegExp", "String". The internal attributes of [[Class]] are used by the engine to determine which type of value an object belongs to. It should be noted that this specification does not provide any other way to allow programs to access the value of the attribute except through methods.
In short, to obtain the real built-in type of an object, we need to obtain the property value of [[Class]]. Before es5, the property value can only be accessed through the source. Therefore, by changing the this pointing of the tostring method (arr), we can obtain the built-in type of the object.
Summarize
The above is an analysis of the reasons why const _toStr = is introduced to you by the editor. I hope it will be helpful to you. If you have any questions, please leave me a message and the editor will reply to you in time. Thank you very much for your support for my website!