如何在JavaScript中正确比较2个字符串？

Question

I am absolutely new in JavaScript and I have a problem comparing 2 strings.

Into a function I have this code:

var selectedProjectStatusText = selectedProjectStatus.options[selectedProjectStatus.selectedIndex].text;

alert("== " + selectedProjectStatusText == "-- Please Select --");
alert("=== " + selectedProjectStatusText === "-- Please Select --");

I am using the alert() function to try how to work string comparator.

The problem is that both the alert() say me FALSE when the selectedProjectStatusText variable contains the -- Please Select -- string.

Why? What am I missing? what could be the cause of the problem? How can I fix it? What am I missing?

Answer 1

You need to remove "=== " + from the start of your conditional statement, or wrap in parenthesis "=== " + (condition) try this instead:

alert(selectedProjectStatusText == "-- Please Select --");

http://jsfiddle.net/eceb5bpx/

If you are wanting to see if the string contains '-- Please Select --' then your condition is incorrect and you should instead use

alert(string.indexOf('-- Please Select --') > -1);

Answer 2

Try putting your condition in parentheses so that it's evaluated before string concatenation.

 var selectedProjectStatusText = "-- Please Select --"; alert("== " + (selectedProjectStatusText == "-- Please Select --")); alert("=== " + (selectedProjectStatusText === "-- Please Select --")); 

Answer 3

You could use the JS concat() function to display the results of your comparison in the format you are using:

See JSFiddle

var selectedProjectStatusText = "-- Please Select --";
alert("== ".concat(selectedProjectStatusText == "-- Please Select --"));
alert("=== ".concat(selectedProjectStatusText === "-- Please Select --"));

"==" : equal to, while "===" : equal value and equal type