为什么'+'运算符在JavaScript中的行为不同于'*'，'/'和'-'运算符

Question

When we perform sum operation between two elements, it needs to be converted into a number, without converting it merges them because element.val() returns a string. But when we perform other operations like multiply, subtraction or divide it gives the expected result without performing any typecast.

 function add() { alert($("#first").val() + $("#second").val()); } function divide() { alert($("#first").val() / $("#second").val()); } function multi() { alert($("#first").val() * $("#second").val()); } function subst() { alert($("#first").val() - $("#second").val()); } 

 <input type="text" Id="first" /> <input type="text" Id="second" /> <input type="button" Id="add" onclick="add()" value="+"/> <input type="button" Id="divide" onclick="divide()" value="/"/> <input type="button" Id="multi" onclick="multi()" value="*" /> <input type="button" Id="subst" onclick="subst()" value="-"/> <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script> 

Answer 1

+ operator is used either for concatenating strings or for adding numbers (the interpretter will favor the concatenation if one of the operands is a string). But / , * and % are used only with numbers so the interpretter is forced to see the operands as numbers.

For more explanation, here is an answer I gave about two days ago for a similar question. (It's pretty long so I won't repost it here again).