How to implement delayed execution in a JavaScript loop?

Question

I want to add a delay/sleep in a while loop:

I tried the following:

alert('hi');

for(var start = 1; start < 10; start ) {
  setTimeout(function () {
    alert('hello');
  }, 3000);
}

Only the first case is correct: after displaying alert('hi'), it will wait for 3 seconds and then display alert('hello' ), but then alert('hello') will be displayed repeatedly.

What I want is that after 3 seconds of showing alert('hello'), it needs to wait another 3 seconds before showing alert(' hello'), and so on.

Answer 1

Since ES7, there is a better way to waitloop:

// 返回一个在“ms”毫秒后解析的Promise
const timer = ms => new Promise(res => setTimeout(res, ms))

async function load () { // 我们需要将循环包装在一个异步函数中才能使其工作
  for (var i = 0; i < 3; i++) {
    console.log(i);
    await timer(3000); // 然后可以等待创建的Promise
  }
}

load();

When the engine reaches the await section, it sets a timeout and pauses execution of the async function . Then, when the timeout completes, execution continues at that point. This is very useful because you can defer (1) nested loops, (2) conditionals, (3) nested functions:

async function task(i) { // 3
  await timer(1000);
  console.log(`Task ${i} done!`);
}

async function main() {
  for(let i = 0; i < 100; i+= 10) {
    for(let j = 0; j < 10; j++) { // 1
      if(j % 2) { // 2
        await task(i + j);
      }
    }
  }
}
    
main();

function timer(ms) { return new Promise(res => setTimeout(res, ms)); }

Reference materials on MDN

Although ES7 is now supported by NodeJS and modern browsers, you may want to transpile it with BabelJS to run everywhere.

Answer 2

setTimeout() The function is non-blocking and returns immediately. So your loop iterates very quickly and triggers the 3 second timeout in quick succession. That's why your first alert pops up after 3 seconds and all other alerts follow continuously without any delay.

You may want to use code similar to:

var i = 1;                  //  将计数器设置为1

function myLoop() {         //  创建一个循环函数
  setTimeout(function() {   //  当调用循环时，调用一个3秒的setTimeout
    console.log('hello');   //  在这里写入您的代码
    i++;                    //  增加计数器
    if (i < 10) {           //  如果计数器小于10，则调用循环函数
      myLoop();             //  ..  再次触发另一个setTimeout()
    }                       //  ..  
  }, 3000)
}

myLoop();                   //  启动循环