Detailed explanation of using Promise to implement traffic lights in JS

This article introduces you to the use of Promise in JS to achieve traffic light effects through example code. In the article, I introduce you to an example of promise usage. Friends who need it can refer to it. I hope it can help you.

Requirements

• Use promise to realize the jump of traffic light color

• After the green light is executed for three seconds

• After the yellow light is executed for four seconds

• The red light is executed for five seconds

html is implemented as follows:

<ul id="traffic" class="">
 <li id="green"></li>
 <li id="yellow"></li>
 <li id="red"></li>
</ul>

Define an empty class, and then operate the corresponding class name in js to achieve related effects.

CSS is implemented as follows:

ul {
 position: absolute;
 width: 200px;
 height: 200px;
 top: 50%;
 left: 50%;
 transform: translate(-50%,-50%);
}
 /*画3个圆代表红绿灯*/
 ul >li {
  width: 40px;
  height: 40px;
  border-radius:50%;
  opacity: 0.2;
  display: inline-block;
 }
 /*执行时改变透明度*/
 ul.red >#red, 
 ul.green >#green,
 ul.yellow >#yellow{
  opacity: 1.0;
 }
 /*红绿灯的三个颜色*/
 #red {background: red;}
 #yellow {background: yellow;}
 #green {background: green;}

Javascript principle:

The promise function is an asynchronous operation function, and the then() method can be used at the end of the function. We can achieve this by setting delayed execution inside the promise function.

js code is as follows:

function timeout(timer){
  return function(){ 
   return new Promise(function(resolve,reject){
   setTimeout(resolve,timer) 
   })  
  }
  }
 var green = timeout(3000);
 var yellow = timeout(4000);
 var red = timeout(5000);
 var traffic = document.getElementById("traffic");
 (function restart(){
  'use strict'      //严格模式
  console.log("绿灯"+new Date().getSeconds()) //绿灯执行三秒 
  traffic.className = 'green';
  green()
  .then(function(){
   console.log("黄灯"+new Date().getSeconds()) //黄灯执行四秒
   traffic.className = 'yellow';
   return yellow();
  })
  .then(function(){
   console.log("红灯"+new Date().getSeconds()) //红灯执行五秒
   traffic.className = 'red';
   return red();
  }).then(function(){
   restart()
  })
  })();

Demo please click here!

ps: Let’s look at an example of Promise usage

Note: If you want the then method to be chainable If the formula continues to be executed, the Promise object must be returned! ! !

'use strict'; 
 
function async(value) { 
  return new Promise(function(resolve, reject) { 
    var ms = Math.round(Math.random() * 1000); 
    setTimeout(function() { 
      console.log('waiting ' + ms + 'ms'); 
      // 等待ms毫秒 
      resolve(value + ms); 
    }, ms); 
  }); 
} 
// 每次执行随机等待n毫秒，结果统计总毫秒数 
async(0) 
.then(async) 
.then(async) 
.then(async) 
.then(async) 
.then(function(value) { 
  console.log('------total wait:' + value + 'ms'); 
}); 
//////////////////////////////////////////////////////////// 
function async1(value) { 
  return new Promise(function(resolve, reject) { 
    resolve(value + 1); 
  }); 
} 
function async2(value) { 
  // return new Promise(function(resolve, reject) { 
  //   resolve(value + 2); 
  // }); 
  // 等价与上面写法 
  return Promise.resolve(value + 2); 
} 
function async3(value) { 
  return new Promise(function(resolve, reject) { 
    resolve(value + 3); 
  }); 
} 
async1(100).then(async2).then(async3).then(function(value) { 
  console.log('------' + value); 
}); 
///////////////////////////////////////////////////////////////// 
function say() { 
  var value = 'what'; 
  return Promise.resolve(value); 
} 
say().then(function(value) { 
  value = value + ' are'; 
  return Promise.resolve(value); 
}).then(function(value) { 
  value = value + ' you'; 
  return Promise.resolve(value); 
}).then(function(value) { 
  value = value + ' doing'; 
  return Promise.resolve(value); 
  //return Promise.reject('error, exit'); // 中途退出 
}).then(function(value) { 
  value = value + ' now!'; 
  return Promise.resolve(value); 
}).then(function(value) { 
  console.log('------' + value); 
}).catch(function(error) { 
  console.log('------' + error); 
}); 
<html> 
<head> 
  <title>Ball move</title> 
  <style type="text/css"> 
    .ball { 
      width: 40px; 
      height: 40px; 
      border-radius: 20px; 
      margin-left: 10px; 
    } 
    .ball1 { 
      background: #ff0000; 
    } 
    .ball2 { 
      background: #00ff00; 
    } 
    .ball3 { 
      background: #0000ff; 
    } 
  </style> 
  <script src="http://libs.baidu.com/jquery/1.9.0/jquery.js"></script> 
</head> 
<body> 
  <p class="ball ball1"></p> 
  <p class="ball ball2"></p> 
  <p class="ball ball3"></p> 
  <script type="text/javascript"> 
    function moving(ball, pos) { 
      return new Promise(function(resolve, reject) { 
        var marginLeft = parseInt(ball.css('margin-left')); 
        if (marginLeft != pos) { 
          var timerId = setInterval(function() { 
            marginLeft = marginLeft + 1; 
            ball.css('margin-left', marginLeft); 
            if (marginLeft == pos) { 
              clearInterval(timerId); 
              resolve(); 
            } 
          }, 20); 
        } else { 
          resolve(); 
        } 
      }); 
    } 
    moving($('.ball1'), 100).then(function() { 
      return moving($('.ball2'), 150); 
    }).then(function() { 
      return moving($('.ball3'), 200); 
    }); 
  </script> 
</body> 
</html>

Related recommendations:

Detailed explanation of the sequential execution of promsie.all and promise in the WeChat applet

Specific usage of jQuery's Promise

Promise, Generator (generator), async (asynchronous) function usage

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