We all know that js code is executed from top to bottom, but let's take a look at the following code;
test();
function test(){
console.log("hello world");
};
If we simply think that js is executed from top to bottom, then the above code should report an error, indicating that test is not a method; but the result is not an error, but the correct output;
So we should know that js execution is divided into two parts
1. Pre-parse; 2. Execution
Pre-parse will process variables and functions defined by var in advance;
That is, the above code will be processed as
function test(){
console.log("hello world");
}
test();
So the call does not report an error;
Let's test var again
alert(a);
var a=1;
At this time, the pop-up is undefined; note that there is no error;
The above code is actually pre-parsed as
var a;
alert(a);
a=1;
So what pops up is an undefined;
When the variable has the same name as the function, the promotion of the variable will be ignored. Let's look at an example.
console.log(test);
function test(){
}
console.log(test);
var test = 123;
console.log(test);
The above code will be preprocessed as
function test(){
}
console.log(test);
console.log(test);
test=123;
console.log(test);
We can see that variable name hoisting is ignored;
Function expressions are not hoisted, such as
test();
var test = function () {
};
At this time, it will report an error; it will prompt that test is not a method;
The preprocessed code is
var test;
test();
test=function(){
};
There is no block-level scope in js;
E.g
if(true){
var a=123;
}
In other languages, such as java, the c# variable a is only valid in the if block-level scope;
But in js there is no such block scope; it will be preprocessed as
var a;
if(true){
a=123；
}