Pre parsing, recursive functions and events in day25 JS 2021.09.16

Pre parse ：（ Variable Promotion ）

The code is parsed before execution , Put the names of variables and functions at the top of the current scope .

summary ：

1. use var Variable of keyword name , Promote the variable name to the front of the current scope , Assignment does not raise

2. The customized function is promoted to the front of the current scope as a whole

3. The function has the same name , The back one will cover the front one

4. Variables and functions have the same name , Function first

Interview questions ：

//1. 
var num = 10;
fn1();
function fn1() {
  console.log(num);
  var num = 20;
}

//2. 
var a = 18;
fn2();
function fn2() {
    var b = 9;
    console.log(a);
    console.log(b);
}

//3.
fn3();
console.log(c);
console.log(b);
console.log(a);
function fn3() {
  var a = b = c = 9;
  console.log(a);
  console.log(b);
  console.log(c);
}

//4. Thinking questions
var a = 4;
console.log(a);
a = 6;
console.log(a);
function a() {
    console.log(' Ha ');
}
a();
a = 10;
console.log(a);

Nesting of functions ：（ Calling function in function ）

Scope ：

1. Variables outside a function are called global variables , It can be used outside and inside functions .

2. Variables inside a function are called local variables , Can only be used inside a function .

Variable access and assignment : Will find... In the current scope , If not, they will go to their superiors Until the overall situation is found

visit ： If there is nothing in the overall situation You're going to report a mistake It will also show undefiend

assignment ： If not in the overall situation You will define a variable by yourself And assign

Recursive function ：（ Is to call yourself in a function ） Cause memory consumption Poor performance

// seek 10 Order sum of , namely ：10+9+8+...+1
function facSum(num){
    if(num == 1){
        return 1;
    }
    return num + facSum(num-1);
}
var res = facSum(10);
console.log(res); // 55

event ：（ The user's actions on the web page , for example ： Mouse click 、 Mouse movement 、 Press the keyboard ）

-1） Through the binding structure Trigger the function to execute the call The name of the binding . Event name = function(){}

- 2） Through the binding structure Trigger the function to execute the call The name of the binding . Event name = The name of the function The parentheses that follow the execution cannot have

Mouse events ：

- onclick Click events

- ondblclick double-click

- onmousemove Mouse migration

- onmouseout Mouse removal There is a change when the mouse moves in and out When removed once, it will change once

- onmousedown The mouse click

- onmouseup The mouse is raised

- oncontextmenu Right click

Keyboard events ： The binding object is window

- onkeydown Press the keyboard

- onkeyup Keyboard up

- onkeypress Keyboard tapping Binding key

Form events ： verification

- onfocus focusing

- onblur Lose focus

- onchange change