为什么javascript中.then()里面的Promise同时被执行？

Question

        const promise1 = new Promise(function(resolve, reject) {
          setTimeout(function() {
            resolve('promise1');
          }, 3000);
        });

        const promise2 = new Promise(function(resolve, reject) {
          setTimeout(function() {
            resolve('promise2');
          }, 3000);
        });

        promise2.then(function(value) {
          console.log(value);
          promise1.then(function(value) {
              console.log(value);
            });
        });

output: *after 3 seconds, both displays at the same time

promise2 promise1

What i'm expecting is, after 3 seconds, promise2 will be displayed first then after another 3 seconds, promise1 will be displayed because promise1 should only execute after the console log in promise2 .then().

Answer 1

Promises are eager in evaluation, you can make them lazy by wrapping them in a Function.

 const promise1 = () => new Promise(function(resolve, reject) { setTimeout(function() { resolve('promise1'); }, 3000); }); const promise2 = () => new Promise(function(resolve, reject) { setTimeout(function() { resolve('promise2'); }, 3000); }); promise2() .then(console.log) .then(() => promise1()) .then(console.log)