为什么我的 Promise 定义会被执行？

Question

I am quite new with Promises and want to know why is it that my Promise definition gets executed without me calling a .then() or resolve on it.

var promise = new Promise(function (resolve, reject) {
     console.log("Starting loader");
     resolve();
});

If you run the sample and see the console you will see the 'Starting loader' message.

https://jsfiddle.net/npqgpcud/

Answer 1

That is simply how promises are defined. They run their executor function immediately. It's in the spec: Promise(executor) , step 9.

This is an instance of the revealing constructor pattern ; reading that might help you understand.

Answer 2

That occurs because a promise will execute immediately and synchronously.

.then() add functions that will be executed when the promise is either fulfilled ( resolve argument) or rejected ( reject argument).

_{with info from comments by @Kirill Slatin}