问题
I am searching for a reason for why increment operator doesn't increment the numeric value in the place where I set the innerHTML value, like below:
<div id="php"></div>
var a = 14;
document.getElementById("php").innerHTML = a++;//the result will be 14 instead of 15
回答1:
It does increase the variable, however it writes to the innerHTML then increments the value, You could use ++a which will increase the value before it writes to the innerHTML.
var a = 14
var b = 14
console.log('before a', a)
document.getElementById("test1").innerHTML = a++
console.log('after a', a)
console.log('before b', b)
document.getElementById("test2").innerHTML = ++b
console.log('after b', b)<div id="test1"></div>
<div id="test2"></div>回答2:
Instead of using a++ you can do ++a to get the increment before the variable value assignation:
var a = 14;
document.getElementById("php").innerHTML = ++a;<div id="php"></div>回答3:
Because you use the postfix increment operator variable++, that means you get the value first and then the variable is incremented.
When you use prefix increment operator ++variable, the variable gets incremented first and then the value is returned.
var a = 42;
console.log(a++); // shows 42, value is 43
console.log(a); // 43
console.log(++a); // 44
console.log(a); // 44来源：https://stackoverflow.com/questions/40950766/post-increment-operator-does-not-increment-variable-until-after-evaluation