JavaScript-reference transfer of object parameters

1. JavaScripthoisting & gt; see the example. We use the Chrome console as the JS runtime environment. Directly execute console. log (a) above and throw the notdefined error without any suspense. This is expected. Let's look at the evolved code below: the variable is not defined before... Syntax 1. JavaScript hoisting> Please refer to the example. We use Chrome console as the JS runtime environment. The console. log (a) is directly executed above, and the not defined error is thrown without any suspense. This is expected. Let's look at the evolved code below: The variable has no defined errors. Instead, it tells us that the value of a is 'undefined '. No matter why the value of a is 'undefined', at least we can know that the variable a is defined, because the previous error of 'a is not defined' is missing. This is precisely because a declaration in JavaScript plays a role in advance. In JavaScript, variables declared in subsequent statements can be used in advance. This feature is called Hoisting by a foreign user (ben cherry ). It can be understood that the declaration of variables is in advance, so no error will be reported if the variable is used before the declaration of variables. In addition, this feature is not limited to variable names, but also has the same effect on function declaration. The first call to function foo () also reports the 'not defined' error. This is both reasonable and legal. Because there is no definition of such a thing as foo () from start to end. Then write the function call at the beginning, but the definition of the function is written at the end.> Let's take a look at the problem that the output value of a is 'undefined. The Hoisting feature needs to be thoroughly understood here. It only advances the declaration in advance, but the assignment of variables is not followed in advance. This is critical. That is why we can use variable a In the first sentence, but its value is 'undefined '. The default value of a variable declared in JavaScript but not assigned a value is 'undefined '. According to Hoisting, the final equivalent code should be like this: 2. you can call the method of string literal directly to call the method of string literal. The JS parser will automatically convert the string variable to a string type for temporary storage, and then call the method on the string, then, the temporary string type is destroyed. So you will see the following usage. We can understand that the parser declares a variable in the background to store this string and then calls the length method.