Variables include: global variables, local variables
In JAvaScript, the variable is a function of local variables defined in the
Scope: is the use of a range of variables,
Divided into: the local scope and global scope
js --- No variable block-level scope defined a pair of brackets, this variable can be used outside the braces
Scope chain: variable is used, from the inside outward, layers of the search, the search can be used directly
var num = 10; // scope chain level: 0
was num2 = 20;
var str = "abc"
function f1() {
was num2 = 20;
function f2() {
was num3 = 30;
console.log(num);
}
f2();
}
f1 ();
Layers of search, the search scope to 0 when or if this variable is not found, the result is an error
Pre-analysis: that is, before the browser parsing code, the function of variable declarations and statements in advance (lift) to the top of the scope
Lift (1) variables
Below this case, the variable is declared in advance, but did not advance the value of num, the result is undefined
// variable lift
console.log(num);
var num = 100;
After // promoted to:
var num; // variables declared in advance
console.log(num);
var num = 100;
（2）
Function declaration is advanced, the code can still perform
// function declaration is ahead
f1 ();
function f1() {
console.log ( "This function, carried out");
}
But for the following scenario, the code error
f2();
var f2=function () {
console.log ( "Xiao Yang Shuaio good");
}
// statement issued after the advance:
var f2; // a variable, undefind
f2 (); // undefind parentheses is not recognized, so the error
var f2=function () {
console.log ( "Xiao Yang Shuaio good");
}
To not being given, the code can be changed to:
where f 2;
f2=function () {
console.log ( "Xiao Yang Shuaio good");
};
f2();