How to detect boolean true and false in javascript?

Question

This is my code for date validate, when fill 28/02/2017 into input type text and press button.

First it's will alert true when see in code if first alert is true secone alert will alert true too. But my second is alert false.

What is my miskate for detect boolean

if(regex.test(txt) === true)
{
    alert("true");
    p.innerHTML = "<h1 style='color:green'>Correct</h1>";
}
else
{
    alert("false");
    p.innerHTML = "<h1 style='color:red'>Wrong</h1>";;
}

.

<input type="text" id="date" name="date" value="dd/mm/yyyy" />
<button onclick="isDate();">Check</button>

<p id="response"></p>
<script>
function isDate() {
    
    var regex = /^(((0[1-9]|[12]\d|3[01])\/(0[13578]|1[02])\/((19|[2-9]\d)\d{2}))|((0[1-9]|[12]\d|30)\/(0[13456789]|1[012])\/((19|[2-9]\d)\d{2}))|((0[1-9]|1\d|2[0-8])\/02\/((19|[2-9]\d)\d{2}))|(29\/02\/((1[6-9]|[2-9]\d)(0[48]|[2468][048]|[13579][26])|((16|[2468][048]|[3579][26])00))))$/g;
    
    var txt = document.getElementById("date").value;
    var p = document.getElementById("response");
    
 alert(regex.test(txt));
 
 if(regex.test(txt) === true)
 {
  alert("true");
  p.innerHTML = "<h1 style='color:green'>Correct</h1>";
 }
 else
 {
  alert("false");
  p.innerHTML = "<h1 style='color:red'>Wrong</h1>";;
 }  
} 
</script>

For some reason, adding console.log below the alert repeating the test, then removing the `===` and `true` it returns true then logs false... VERY strange!! spoookky — L_Church, Apr 26 '18 at 15:29

score 7 · Accepted Answer · edited Apr 26 '18 at 15:48

7

The reason is that regex.test() will search the string and return true if there is a match. If you then subsequently call .test() again it will continue from where it left off.

Using test() on a regex with the global flag

If the regex has the global flag set, test() will advance the lastIndex of the regex. A subsequent use of test() will start the search at the substring of str specified by lastIndex (exec() will also advance the lastIndex property).

From here: MDN web docs – RegExp.prototype.test()

edited Apr 26 '18 at 15:48

Rory O'Kane

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answered Apr 26 '18 at 15:32

Reinstate Monica Cellio

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1

work good sir.. – reswe teryuio Apr 26 '18 at 15:35
Just a question, why it is working if you remove g modifier? thanks! – Albeis Apr 26 '18 at 15:39
@Albeis If you remove the global flag it searches from the beginning every time. – Reinstate Monica Cellio Apr 26 '18 at 15:40
Thanks! so removing global flag should be another solution right? regards – Albeis Apr 26 '18 at 15:41
@Albeis ‘g’ stands for ‘global’ – the `g` modifier sets the `global` flag. And that affects the “If the regex has the global flag set” behavior described in the documentation. – Rory O'Kane Apr 26 '18 at 15:41
@Albeis It depends on the string you're testing. In this specific instance then removing the global flag would fix the issue, yes. – Reinstate Monica Cellio Apr 26 '18 at 15:42
@Archer yes, on this specific instance. regards – Albeis Apr 26 '18 at 15:45

How to detect boolean true and false in javascript?

1 Answers1

Using `test()` on a regex with the global flag

How to detect boolean true and false in javascript?

1 Answers1

Using test() on a regex with the global flag

Using `test()` on a regex with the global flag