Javascript如何知道变量的类型？ [英] How does Javascript know what type a variable is?
问题描述
我不知道为什么我从未问过自己过去几年的质疑，但突然间我找不到自己或谷歌的任何答案。
I don't know why I never asked myself that questioned the last years before, but suddenly I could not find any answer for myself or with google.
Javascript众所周知，变量没有类型。真是件好事。
但不知何故，它必须确定类型并使用它。
Javascript is known to have no types for variables. A really nice thing. But somehow it must determine the type and work with it.
var a = 1;
var b = 2.0;
var c = 'c';
var d = "Hello World!";
所以我们有一个整数，双/浮点数，字符，字符串（可能会被拆除） as char *）
So what we have is an Integer, Double/Float, Character, String (which may be teared down as char*)
我知道JS使用运行时解释器，但是想到逻辑和类型必须以任何方式实现..
I know that JS works with a runtime interpreter, but thinking of that the logic and "type" must be implemented in any way..
那么Javascript Interpreter如何识别并在内部处理变量？
在我的想象中，假设我会编写C ++，我会想到一种模板和容器以及一些逻辑，它会重载运算符并尝试检查它到底是什么。但这并没有想到最后。
So how does a Javascript Interpreter recognize and internally handle the variables? In my imagination, assuming I would write C++, I would think of a sort of template and container and a bit of a logic that overloads operators and try to check, what it really is. But that's not thought to the end.
请与我分享你的知识： - ）
Share your knowledge with me please :-)
推荐答案
JavaScript根据值赋值设置变量类型。例如，当JavaScript遇到以下代码时，它知道 myVariable 应该是类型编号：
JavaScript sets the variable type based on the value assignment. For example when JavaScript encounters the following code it knows that myVariable should be of type number:
var myVariable = 10;
同样，JavaScript将在以下示例中检测到变量类型是字符串：
Similarly, JavaScript will detect in the following example that the variable type is string:
var myVariable = "Hello World!";
JavaScript比许多其他编程语言灵活得多。对于Java等语言，必须在创建变量时将变量声明为特定类型，并且一旦创建，就不能更改该类型。这被称为 强打字 。另一方面，JavaScript允许随时更改变量的类型，只需指定不同类型的值（更好地称为 松散的打字 ）。
JavaScript is also much more flexible than many other programming languages. With languages such as Java a variable must be declared to be a particular type when it is created and once created, the type cannot be changed. This is referred to as strong typing. JavaScript, on the other hand, allows the type of a variable to be changed at any time simply by assigning a value of a different type (better known as loose typing).
以下示例完全有效地使用了JavaScript中的变量。在创建时，变量显然是类型编号。稍后将字符串赋值给此变量会将类型从数字更改为字符串。
The following example is perfectly valid use of a variable in JavaScript. At creation time, the variable is clearly of type number. A later assignment of a string to this variable changes the type from number to string.
var myVariable = 10;
myVariable = "This is now a string type variable";
变量的数据类型是JavaScript脚本引擎的解释类型变量当前持有的数据。字符串变量包含字符串;数字变量包含数字值，依此类推。但是，与许多其他语言不同，在JavaScript中，同一个变量可以包含不同类型的数据，所有这些都在同一个应用程序中。这是松散类型和动态类型这两个术语所知的概念，这两个概念都意味着JavaScript变量可以根据上下文在不同时间保存不同的数据类型。
The variable’s data type is the JavaScript scripting engine’s interpretation of the type of data that variable is currently holding. A string variable holds a string; a number variable holds a number value, and so on. However, unlike many other languages, in JavaScript, the same variable can hold different types of data, all within the same application. This is a concept known by the terms loose typing and dynamic typing, both of which mean that a JavaScript variable can hold different data types at different times depending on context.
完整文章： http://www.techotopia.com/index.php/JavaScript_Variable_Types
Complete article here: http://www.techotopia.com/index.php/JavaScript_Variable_Types
另一篇文章可以帮助您： http://oreilly.com/javascript/excerpts/learning-javascript/javascript-datatypes-variables.html
Another Article Which may help you: http://oreilly.com/javascript/excerpts/learning-javascript/javascript-datatypes-variables.html
有用的链接：
ECMAScript Language Specification
ECMAScript BNF Grammar
JAVAScript BNF Gramar
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