Javascript“eval”是同步还是异步？

Question

Consider the following code:

eval(".....;a=5;b=10;");
eval("a+b");

If in case here the 1st eval runs for long time, will my next eval return an error mentioning as a and b are undefined, as the a and b values are initialised at the end of 1st eval. Will the eval method run synchronously or asynchronously

Answer 1

eval is synchronous.

Lets look at this example:

 console.log("before") eval("console.log('eval')"); console.log("after"); 

You can see in that the print is in the order.

if it was asynchronous for example in this case:

 console.log("before"); setTimeout(()=>console.log("asynchronous"),0) console.log("after") 

The asynchronous run after.

Answer 2

eval is synchronous in nature. But the evaluations/expressions inside the eval may have asynchronous code like setTimeout or setInterval .

For an instance.

Method 1 : (synchronous example)

 eval('var a=5, b=10;'); eval('console.log(a+b)'); 

Method 2 : (asynchronous example)

 eval('setInterval(function(){window["a"]=5, window["b"]=10;}, 1000)'); eval('console.log(typeof a)'); 

Note : Anyways, it's not recommended to use eval as mentioned in https://stackoverflow.com/a/86580/7055233