1.声明方式
函数声明存在提升,函数表达式不存在提升
函数声明
alert(sum(10,10));//20
function sum(num1, num2){
return num1 + num2;
};
函数表达式
alert(sum(10,10)); //sum is not a function
var sum = function(num1, num2){
return num1 + num2;
};
2. 函数的参数
function sum(a,b,c){//a,b,c为形参
if(sum.length > arguments.length){//arguments为实参列表,类型是伪数组
console.log("形参多")
}else{
console.log("实参多")
}
}
sum(1,2);//1,2为实参
function sum(a,b,c){
arguments[1] = 333;
c = 999;
console.log(arguments[1]);//333
console.log(b);//333
console.log(arguments[2]);//undefined
}
sum(1,2);
3.函数预编译
- 创建AO对象 Activation Object(执行期上下文,也就是作用域)
- 找形参和变量声明,将变量和形参名作为AO属性名,值为undefined
- 将实参值和形参统一
- 在函数体里面找到函数声明,值赋予函数体
function fn(a){
console.log(a);//ƒ a(){}
var a = 123;
console.log(a);//123
function a(){}
console.log(a);//123
var b = function(){}
console.log(b);//ƒ (){}
}
function test(a,b){
console.log(a);//1
c = 0;
var c;
a = 3;
b = 2;
console.log(b);//2
function b(){}
function d(){}
console.log(a);//3
console.log(b);//2
}
test(1);
function test(a,b){
console.log(a);//ƒ a(){}
console.log(b);//undefined
var b = 234;
console.log(b);//234
a = 123;
console.log(a);//123
function a(){}
var a;
b = 234;
var b = function(){}
console.log(a);//123
console.log(b);//ƒ (){}
}
test(1);
4.全局预编译
- 创建GO对象 Global Object(执行期上下文,也就是作用域)
- 找全局的变量声明,将变量名作为GO属性名,值为undefined
- 在全局 里面找到函数声明,值赋予函数体
global = 100;
function fn() {
console.log(global);//undefined
global = 200;
console.log(global);//200
var global = 300;
}
fn();
var global;