javascript - timer anonymous function problem

Question

var a = 0;
function fn(){
    ssss.call(null,a)
    // 为什么 定时器里面的匿名函数加上字符串跟直接执行不同？？？
    setInterval("ssss()", 1000);    
    setInterval(ssss(), 1000);
}
function ssss(){
    console.log(++a)
}
fn();

Please ask the master to explain the principle behind it

Answer 1

The first parameter of setInterval accepts a string and will parse the string into a function statement for execution.

Answer 2

First, let’s take a look at W3C’s explanation of setInterval

and then look at it

1. setInterval("ssss()", 1000);

2. setInterval(ssss(), 1000);

1. Then an error will be reported when executing;
2. Function body ssss()

function ssss(){
    console.log(++a)
}

There is no return value, but note that there is a sentence ssss.call(null,a) in the fn function, so there is a return value in the fn function. Moreover, the return value is just a function, so it meets the function requirements of setInterval and will continue to be executed

Answer 3

If you don’t add double quotes, you need to remove the parentheses and just write the function name