Understanding JavaScript pre-parsing_javascript skills
May 16, 2016 pm 06:43 PMIn fact or some phenomenon proves that this is not the case. Through the "JavaScript Definitive Guide" and related online information, we learned that JavaScript has "pre-parsing" behavior. It is important to understand this feature, otherwise you may encounter many problems that cannot be resolved during actual development, and even lead to program bugs. In order to analyze this phenomenon, and as a summary of my own learning, this article will gradually guide you to understand JavaScript "pre-parsing". If my opinion is wrong, please correct me.
(1) If JavaScript is only parsed from top to bottom at runtime, it is understandable that the following code can run correctly, because we first define the function and then call it.
function showMsg()
{
alert ('This is message');
}
showMsg(); // This is message
(2) We also know that functions can be defined after the calling code, as is the following code Can work normally. It seems that showMsg() is still not defined when showMsg() is called, but it works normally, which indicates that JavaScript is "pre-parsed".
showMsg(); // This is message
function showMsg()
{
alert('This is message');
}
(3) The above is an example of a function, and here is another example of an ordinary variable. When the following example is run, undefined will pop up, indicating that msg in the first sentence has been defined, but has not been initialized. It is the same as var msg; alert(msg);. If you comment out the second sentence below, an "msg is not defined" error will be reported. This also indicates that JavaScript is "pre-parsed".
alert(msg); //undefined
var msg='This is message';
(4) Let’s look at another example to deepen our impression of JavaScript “pre-parsing”. In the following code, you will see that the dialog box that pops up twice displays This is message 2. Why is this? In fact, two functions with the same name are defined one after another below. The later showMsg() overwrites the previously defined one (in JavaScript, variables with the same name will also have overwriting problems), which means that the first showMsg() is scrapped. Why does the second call of showMsg() not call the message 1 function defined above? This once again proves that JavaScript has "preparsing" behavior.
showMsg(); // This is message 2
function showMsg()
{
alert('This is message 1');
}
showMsg(); // This is message 2
function showMsg()
{
alert('This is message 2');
}
(5) JavaScript "pre-parsing" is to pre-parse variables or functions into the environment where they can be called (variable execution environment). The following code seems to have seen the definition of msg before alert(msg), but the program still reports the error "msg is not defined". This is because the variables defined in the function are private variables of the function and cannot be called directly from outside. This indicates that JavaScript "Pre-parsing" does not mean parsing all defined variables into a global object, such as window.
function showMsg()
{
var msg='This is message';
}
alert(msg); // msg is undefined
(6) JavaScript "pre-parsing" is performed in sections, to be precise It is divided into <script> blocks. The following code appears in two script blocks on the same page and defines three functions with the same name. The program running results show that showMsg() of the second script block does not cover the first two showMsg(), while the second showMsg() of the first script block covers the first showMsg().
<body>
<script type= "text/javascript">
showMsg(); //This is message 2
function showMsg()
{
alert('This is message 1');
}
function showMsg()
{
alert('This is message 2');
}
</script>
<script type="text/javascript">
function showMsg()
{
alert('This is message 3');
}
</script>
</body>
Author: WebFlash
Source: http://webflash.cnblogs.com
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