练习题

第1题

console.log(a);
var a = 3;    // undefined
-----------------------
console.log(a);
a = 3;    // throw ReferenceError

第2题：error

foo();    // TypeError
bar();    // ReferenceError
var foo=function bar()    {...}

第3题：闭包

for (var i = 0; i < 10; i++) {  
    setTimeout(function () {
    alert('I am link #' + i);
  }, 100);
}

第4题: 函数

var a=1
var b=2; 
function test(a, b){
    var temp = a; 
    a = b; 
    b = temp; 
} 
test(a, b); 

console.log('a---'+ a);
console.log('b---'+ b);

var a={a: 1}
var b={b: 1}; 
function test(a, b){
    var temp = a; 
    a = b; 
    b = temp; 
} 
test(a, b); 

console.log('a---'+ JSON.stringify(a));
console.log('b---'+ JSON.stringify(b));

第5题： event loop

console.log('start')

setTimeout(() => {
  console.log('setTimeout')
}, 0)

new Promise((resolve) => {
  console.log('promise')
  resolve()
})
  .then(() => {
    console.log('then1')
  })
  .then(() => {
    console.log('then2')
  })

console.log('end')

第5题： event loop 2

console.log('script start')

async function async1() {
  await async2()
  console.log('async1 end')
}
async function async2() {
  console.log('async2 end')
}
async1()

setTimeout(function() {
  console.log('setTimeout')
}, 0)

new Promise(resolve => {
  console.log('Promise')
  resolve()
})
  .then(function() {
    console.log('promise1')
  })
  .then(function() {
    console.log('promise2')
  })

console.log('script end')